Php – My Routes are Returning a 404, How to Fix Them

The correct way to avoid SQL injection attacks, no matter which database you use, is to separate the data from SQL, so that data stays data and will never be interpreted as commands by the SQL parser. It is possible to create SQL statement with correctly formatted data parts, but if you don't fully understand the details, you should always use prepared statements and parameterized queries. These are SQL statements that are sent to and parsed by the database server separately from any parameters. This way it is impossible for an attacker to inject malicious SQL.

You basically have two options to achieve this:

1. Using PDO (for any supported database driver):

    $stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');
   
    $stmt->execute([ 'name' => $name ]);
   
    foreach ($stmt as $row) {
        // Do something with $row
    }
   

2. Using MySQLi (for MySQL):

    $stmt = $dbConnection->prepare('SELECT * FROM employees WHERE name = ?');
    $stmt->bind_param('s', $name); // 's' specifies the variable type => 'string'
   
    $stmt->execute();
   
    $result = $stmt->get_result();
    while ($row = $result->fetch_assoc()) {
        // Do something with $row
    }
   

If you're connecting to a database other than MySQL, there is a driver-specific second option that you can refer to (for example, pg_prepare() and pg_execute() for PostgreSQL). PDO is the universal option.

Correctly setting up the connection

Note that when using PDO to access a MySQL database real prepared statements are not used by default. To fix this you have to disable the emulation of prepared statements. An example of creating a connection using PDO is:

$dbConnection = new PDO('mysql:dbname=dbtest;host=127.0.0.1;charset=utf8', 'user', 'password');

$dbConnection->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

In the above example the error mode isn't strictly necessary, but it is advised to add it. This way the script will not stop with a Fatal Error when something goes wrong. And it gives the developer the chance to catch any error(s) which are thrown as PDOExceptions.

What is mandatory, however, is the first setAttribute() line, which tells PDO to disable emulated prepared statements and use real prepared statements. This makes sure the statement and the values aren't parsed by PHP before sending it to the MySQL server (giving a possible attacker no chance to inject malicious SQL).

Although you can set the charset in the options of the constructor, it's important to note that 'older' versions of PHP (before 5.3.6) silently ignored the charset parameter in the DSN.

Explanation

The SQL statement you pass to prepare is parsed and compiled by the database server. By specifying parameters (either a ? or a named parameter like :name in the example above) you tell the database engine where you want to filter on. Then when you call execute, the prepared statement is combined with the parameter values you specify.

The important thing here is that the parameter values are combined with the compiled statement, not an SQL string. SQL injection works by tricking the script into including malicious strings when it creates SQL to send to the database. So by sending the actual SQL separately from the parameters, you limit the risk of ending up with something you didn't intend.

Any parameters you send when using a prepared statement will just be treated as strings (although the database engine may do some optimization so parameters may end up as numbers too, of course). In the example above, if the $name variable contains 'Sarah'; DELETE FROM employees the result would simply be a search for the string "'Sarah'; DELETE FROM employees", and you will not end up with an empty table.

Another benefit of using prepared statements is that if you execute the same statement many times in the same session it will only be parsed and compiled once, giving you some speed gains.

Oh, and since you asked about how to do it for an insert, here's an example (using PDO):

$preparedStatement = $db->prepare('INSERT INTO table (column) VALUES (:column)');

$preparedStatement->execute([ 'column' => $unsafeValue ]);

Can prepared statements be used for dynamic queries?

While you can still use prepared statements for the query parameters, the structure of the dynamic query itself cannot be parametrized and certain query features cannot be parametrized.

For these specific scenarios, the best thing to do is use a whitelist filter that restricts the possible values.

// Value whitelist
// $dir can only be 'DESC', otherwise it will be 'ASC'
if (empty($dir) || $dir !== 'DESC') {
   $dir = 'ASC';
}

No output before sending headers!

Functions that send/modify HTTP headers must be invoked before any output is made. summary ⇊ Otherwise the call fails:

Warning: Cannot modify header information - headers already sent (output started at script:line)

Some functions modifying the HTTP header are:

• header / header_remove
• session_start / session_regenerate_id
• setcookie / setrawcookie

Output can be:

• Unintentional:

  • Whitespace before <?php or after ?>
  • The UTF-8 Byte Order Mark specifically
  • Previous error messages or notices

• Intentional:

  • print, echo and other functions producing output
  • Raw <html> sections prior <?php code.

Why does it happen?

To understand why headers must be sent before output it's necessary to look at a typical HTTP response. PHP scripts mainly generate HTML content, but also pass a set of HTTP/CGI headers to the webserver:

HTTP/1.1 200 OK
Powered-By: PHP/5.3.7
Vary: Accept-Encoding
Content-Type: text/html; charset=utf-8

<html><head><title>PHP page output page</title></head>
<body><h1>Content</h1> <p>Some more output follows...</p>
and <a href="/"> <img src=internal-icon-delayed> </a>

The page/output always follows the headers. PHP has to pass the headers to the webserver first. It can only do that once. After the double linebreak it can nevermore amend them.

When PHP receives the first output (print, echo, <html>) it will flush all collected headers. Afterward it can send all the output it wants. But sending further HTTP headers is impossible then.

How can you find out where the premature output occurred?

The header() warning contains all relevant information to locate the problem cause:

Warning: Cannot modify header information - headers already sent by (output started at /www/usr2345/htdocs/auth.php:52) in /www/usr2345/htdocs/index.php on line 100

Here "line 100" refers to the script where the header() invocation failed.

The "output started at" note within the parenthesis is more significant. It denominates the source of previous output. In this example, it's auth.php and line 52. That's where you had to look for premature output.

Typical causes:

1. Print, echo

   Intentional output from print and echo statements will terminate the opportunity to send HTTP headers. The application flow must be restructured to avoid that. Use functions and templating schemes. Ensure header() calls occur before messages are written out.

   Functions that produce output include

   • print, echo, printf, vprintf
   • trigger_error, ob_flush, ob_end_flush, var_dump, print_r
   • readfile, passthru, flush, imagepng, imagejpeg

   
   among others and user-defined functions.

2. Raw HTML areas

   Unparsed HTML sections in a .php file are direct output as well. Script conditions that will trigger a header() call must be noted before any raw <html> blocks.

   <!DOCTYPE html>
   <?php
       // Too late for headers already.
   

   Use a templating scheme to separate processing from output logic.

   • Place form processing code atop scripts.
   • Use temporary string variables to defer messages.
   • The actual output logic and intermixed HTML output should follow last.
     
     

3. Whitespace before <?php for "script.php line 1" warnings

   If the warning refers to output inline 1, then it's mostly leading whitespace, text or HTML before the opening <?php token.

    <?php
   # There's a SINGLE space/newline before <? - Which already seals it.
   

   Similarly it can occur for appended scripts or script sections:

   ?>
   
   <?php
   

   PHP actually eats up a single linebreak after close tags. But it won't compensate multiple newlines or tabs or spaces shifted into such gaps.

4. UTF-8 BOM

   Linebreaks and spaces alone can be a problem. But there are also "invisible" character sequences that can cause this. Most famously the UTF-8 BOM (Byte-Order-Mark) which isn't displayed by most text editors. It's the byte sequence EF BB BF, which is optional and redundant for UTF-8 encoded documents. PHP however has to treat it as raw output. It may show up as the characters  in the output (if the client interprets the document as Latin-1) or similar "garbage".

   In particular graphical editors and Java-based IDEs are oblivious to its presence. They don't visualize it (obliged by the Unicode standard). Most programmer and console editors however do:

   

   There it's easy to recognize the problem early on. Other editors may identify its presence in a file/settings menu (Notepad++ on Windows can identify and remedy the problem), Another option to inspect the BOMs presence is resorting to an hexeditor. On *nix systems hexdump is usually available, if not a graphical variant which simplifies auditing these and other issues:

   

   An easy fix is to set the text editor to save files as "UTF-8 (no BOM)" or similar to such nomenclature. Often newcomers otherwise resort to creating new files and just copy&pasting the previous code back in.

   Correction utilities

   There are also automated tools to examine and rewrite text files (sed/awk or recode). For PHP specifically there's the phptags tag tidier. It rewrites close and open tags into long and short forms, but also easily fixes leading and trailing whitespace, Unicode and UTF-x BOM issues:

   phptags  --whitespace  *.php
   

   It's safe to use on a whole include or project directory.

5. Whitespace after ?>

   If the error source is mentioned as behind the closing ?> then this is where some whitespace or the raw text got written out. The PHP end marker does not terminate script execution at this point. Any text/space characters after it will be written out as page content still.

   It's commonly advised, in particular to newcomers, that trailing ?> PHP close tags should be omitted. This eschews a small portion of these cases. (Quite commonly include()d scripts are the culprit.)

6. Error source mentioned as "Unknown on line 0"

   It's typically a PHP extension or php.ini setting if no error source is concretized.

   • It's occasionally the gzip stream encoding setting or the ob_gzhandler.
   • But it could also be any doubly loaded extension= module generating an implicit PHP startup/warning message.
     
     

7. Preceding error messages

   If another PHP statement or expression causes a warning message or notice being printed out, that also counts as premature output.

   In this case you need to eschew the error, delay the statement execution, or suppress the message with e.g. isset() or @() - when either doesn't obstruct debugging later on.

No error message

If you have error_reporting or display_errors disabled per php.ini, then no warning will show up. But ignoring errors won't make the problem go away. Headers still can't be sent after premature output.

So when header("Location: ...") redirects silently fail it's very advisable to probe for warnings. Reenable them with two simple commands atop the invocation script:

error_reporting(E_ALL);
ini_set("display_errors", 1);

Or set_error_handler("var_dump"); if all else fails.

Speaking of redirect headers, you should often use an idiom like this for final code paths:

exit(header("Location: /finished.html"));

Preferably even a utility function, which prints a user message in case of header() failures.

Output buffering as a workaround

PHPs output buffering is a workaround to alleviate this issue. It often works reliably, but shouldn't substitute for proper application structuring and separating output from control logic. Its actual purpose is minimizing chunked transfers to the webserver.

1. The output_buffering= setting nevertheless can help. Configure it in the php.ini or via .htaccess or even .user.ini on modern FPM/FastCGI setups.
   Enabling it will allow PHP to buffer output instead of passing it to the webserver instantly. PHP thus can aggregate HTTP headers.

2. It can likewise be engaged with a call to ob_start(); atop the invocation script. Which however is less reliable for multiple reasons:

   • Even if <?php ob_start(); ?> starts the first script, whitespace or a BOM might get shuffled before, rendering it ineffective.

   • It can conceal whitespace for HTML output. But as soon as the application logic attempts to send binary content (a generated image for example), the buffered extraneous output becomes a problem. (Necessitating ob_clean() as a further workaround.)

   • The buffer is limited in size, and can easily overrun when left to defaults. And that's not a rare occurrence either, difficult to track down when it happens.

Both approaches therefore may become unreliable - in particular when switching between development setups and/or production servers. This is why output buffering is widely considered just a crutch / strictly a workaround.

See also the basic usage example in the manual, and for more pros and cons:

• What is output buffering?
• Why use output buffering in PHP?
• Is using output buffering considered a bad practice?
• Use case for output buffering as the correct solution to "headers already sent"

But it worked on the other server!?

If you didn't get the headers warning before, then the output buffering php.ini setting has changed. It's likely unconfigured on the current/new server.

Checking with headers_sent()

You can always use headers_sent() to probe if it's still possible to... send headers. Which is useful to conditionally print info or apply other fallback logic.

if (headers_sent()) {
    die("Redirect failed. Please click on this link: <a href=...>");
}
else{
    exit(header("Location: /user.php"));
}

Useful fallback workarounds are:

• HTML <meta> tag

  If your application is structurally hard to fix, then an easy (but somewhat unprofessional) way to allow redirects is injecting a HTML <meta> tag. A redirect can be achieved with:

   <meta http-equiv="Location" content="http://example.com/">
  

  Or with a short delay:

   <meta http-equiv="Refresh" content="2; url=../target.html">
  

  This leads to non-valid HTML when utilized past the <head> section. Most browsers still accept it.

• JavaScript redirect

  As alternative a JavaScript redirect can be used for page redirects:

   <script> location.replace("target.html"); </script>
  

  While this is often more HTML compliant than the <meta> workaround, it incurs a reliance on JavaScript-capable clients.

Both approaches however make acceptable fallbacks when genuine HTTP header() calls fail. Ideally you'd always combine this with a user-friendly message and clickable link as last resort. (Which for instance is what the http_redirect() PECL extension does.)

Why setcookie() and session_start() are also affected

Both setcookie() and session_start() need to send a Set-Cookie: HTTP header. The same conditions therefore apply, and similar error messages will be generated for premature output situations.

(Of course, they're furthermore affected by disabled cookies in the browser or even proxy issues. The session functionality obviously also depends on free disk space and other php.ini settings, etc.)

Further links

• Google provides a lengthy list of similar discussions.
• And of course many specific cases have been covered on Stack Overflow as well.
• The WordPress FAQ explains How do I solve the Headers already sent warning problem? in a generic manner.
• Adobe Community: PHP development: why redirects don't work (headers already sent)
• Nucleus FAQ: What does "page headers already sent" mean?
• One of the more thorough explanations is HTTP Headers and the PHP header() Function - A tutorial by NicholasSolutions (Internet Archive link). It covers HTTP in detail and gives a few guidelines for rewriting scripts.