str_replace 只替换一次

<?php 
function str_replace_once($needle,$haystack) { 


$pos = strpos($haystack, $needle); 
if ($pos === false) { 
return $haystack; 
} 
return substr_replace($haystack, $replace, $pos, strlen($needle)); 
} 


echo str_replace_once($needle,$replace,$haystack);
?>