Short Answer

Use $this to refer to the current object. Use self to refer to the current class. In other words, use $this->member for non-static members, use self::$member for static members.

Full Answer

Here is an example of correct usage of $this and self for non-static and static member variables:

<?php
class X {
    private $non_static_member = 1;
    private static $static_member = 2;

    function __construct() {
        echo $this->non_static_member . ' '
           . self::$static_member;
    }
}

new X();
?>

Here is an example of incorrect usage of $this and self for non-static and static member variables:

<?php
class X {
    private $non_static_member = 1;
    private static $static_member = 2;

    function __construct() {
        echo self::$non_static_member . ' '
           . $this->static_member;
    }
}

new X();
?>

Here is an example of polymorphism with $this for member functions:

<?php
class X {
    function foo() {
        echo 'X::foo()';
    }

    function bar() {
        $this->foo();
    }
}

class Y extends X {
    function foo() {
        echo 'Y::foo()';
    }
}

$x = new Y();
$x->bar();
?>

Here is an example of suppressing polymorphic behaviour by using self for member functions:

<?php
class X {
    function foo() {
        echo 'X::foo()';
    }

    function bar() {
        self::foo();
    }
}

class Y extends X {
    function foo() {
        echo 'Y::foo()';
    }
}

$x = new Y();
$x->bar();
?>

The idea is that $this->foo() calls the foo() member function of whatever is the exact type of the current object. If the object is of type X, it thus calls X::foo(). If the object is of type Y, it calls Y::foo(). But with self::foo(), X::foo() is always called.

From http://www.phpbuilder.com/board/showthread.php?t=10354489:

By http://board.phpbuilder.com/member.php?145249-laserlight

Using some of your code and the original function, I found out a way where it doesn't add an unneccessary blank page at the end of the file.

public function AcceptPageBreak() {
         if (1 == $this->PageNo()) {
                $this->SetMargins($left_margin, $top_margin, $right_margin, true);
         }
        if ($this->num_columns > 1) {
            // multi column mode
            if ($this->current_column < ($this->num_columns - 1)) {
                // go to next column
                $this->selectColumn($this->current_column + 1);
            } elseif ($this->AutoPageBreak) {
                // add a new page
                $this->AddPage();
                // set first column
                $this->selectColumn(0);
            }
            // avoid page breaking from checkPageBreak()
            return false;
        }
        return $this->AutoPageBreak;
    }