PHP seems to consider \0 (NULL chracter) a valid value.
As the result check such as if($val) is true.

Example Code:
<?
$ret_val = exec("ps -p 1 | grep shit");
if( $ret_val ) {
	echo "this shouldn't happen\n";
}
 
echo ord($ret_val)."\n";
echo strlen($ret_val)."\n";
?>

Maybe the type juggling mechanism gets confused?