Description:
------------
The documentation for substr_compare() indicates that the default value for the $length parameter is the string length of the $main_str parameter, but this doesn't appear to be the case.

Reproduce code:
---------------
---
From manual page: function.substr-compare#Description
---
<?php
        $a = "abc123";
        $b = "abc";

        echo "Is the documentation correct? ";
        echo ((substr_compare($b, $a, 0) == (substr_compare($b, $a, 0, strlen($b)))) ? 'true' : 'false');
        echo "\n";
?>


Expected result:
----------------
Is the documentation correct? true

Actual result:
--------------
Is the documentation correct? false