PHP file locking ensures multi-threaded writing safety

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    function write_file($filename, $content)  
    {  
        $lock = $filename . '.lck';  
        $write_length = 0;  
        while(true) {  
            if( file_exists($lock) ) {  
                usleep(100);  
            } else {  
                touch($lock);  
                $write_length = file_put_contents($filename, $content, FILE_APPEND);  
                break;  
            }  
        }  
        if( file_exists($lock) ) {  
            unlink($lock);  
        }  
        return $write_length;  
    }