Description:
------------
For "big" string the substr_count function returns incorrect result.
Memory limit in PHP set to 30GB.
Seems it works incorrect for strings>4GB.

Test script:
---------------
<?php
$size=9*1024*1024*1024;		// 9GB
$buf=str_repeat(chr(0),$size);
for( $i=0; $i<$size; $i++ )
	if( $i%3==0 )
		$buf{$i}=chr(255);
$cnt=substr_count($buf,chr(255));
var_dump($cnt);
?>

Expected result:
----------------
int(3221225472)

Actual result:
--------------
int(-1073741824)