function str_replace_once($needle , $replace , $haystack){ 
    // Looks for the first occurence of $needle in $haystack 
    // and replaces it with $replace. 
    $pos = strpos($haystack, $needle); 
    if ($pos === false) { 
        // Nothing found 
    return $haystack; 
    } 
    return substr_replace($haystack, $replace, $pos, strlen($needle)); 
}