Php – No error messages returned when thesql_connect does not work for PHP on IIS

Warning : mysql_xx functions are deprecated since php 5.5 and removed since php 7.0 (see http://php.net/manual/intro.mysql.php), use mysqli_xx functions or see the answer below from @Troelskn

You can make multiple calls to mysql_connect(), but if the parameters are the same you need to pass true for the '$new_link' (fourth) parameter, otherwise the same connection is reused. For example:

$dbh1 = mysql_connect($hostname, $username, $password); 
$dbh2 = mysql_connect($hostname, $username, $password, true); 

mysql_select_db('database1', $dbh1);
mysql_select_db('database2', $dbh2);

Then to query database 1 pass the first link identifier:

mysql_query('select * from tablename', $dbh1);

and for database 2 pass the second:

mysql_query('select * from tablename', $dbh2);

If you do not pass a link identifier then the last connection created is used (in this case the one represented by $dbh2) e.g.:

mysql_query('select * from tablename');

Other options

If the MySQL user has access to both databases and they are on the same host (i.e. both DBs are accessible from the same connection) you could:

• Keep one connection open and call mysql_select_db() to swap between as necessary. I am not sure this is a clean solution and you could end up querying the wrong database.
• Specify the database name when you reference tables within your queries (e.g. SELECT * FROM database2.tablename). This is likely to be a pain to implement.

Also please read troelskn's answer because that is a better approach if you are able to use PDO rather than the older extensions.

For syntax errors, you need to enable error display in the php.ini. By default these are turned off because you don't want a "customer" seeing the error messages. Check this page in the PHP documentation for information on the 2 directives: error_reporting and display_errors. display_errors is probably the one you want to change. If you can't modify the php.ini, you can also add the following lines to an .htaccess file:

php_flag  display_errors        on
php_value error_reporting       2039

You may want to consider using the value of E_ALL (as mentioned by Gumbo) for your version of PHP for error_reporting to get all of the errors. more info

3 other items: (1) You can check the error log file as it will have all of the errors (unless logging has been disabled). (2) Adding the following 2 lines will help you debug errors that are not syntax errors:

error_reporting(-1);
ini_set('display_errors', 'On');

(3) Another option is to use an editor that checks for errors when you type, such as PhpEd. PhpEd also comes with a debugger which can provide more detailed information. (The PhpEd debugger is very similar to xdebug and integrates directly into the editor so you use 1 program to do everything.)

Cartman's link is also very good: http://www.ibm.com/developerworks/library/os-debug/