optimal way to prepend a file in php

Prepend a string to a file in php

• Size of file can be very large.
• Need an optimal solution while keeping performance as the key factor.

This is what i have so far

    function prepend($string, $filename) { 
        $context = stream_context_create (); 
        $fp = fopen($filename,'r',1,$context); 
        $tmpnam开发者_如何学Ce = md5($string); 
        file_put_contents($tmpname,$string); 
        file_put_contents($tmpname,$fp,FILE_APPEND); 
        fclose($fp); 
        unlink($filename); 
        rename($tmpname,$filename); 
    }