es curioso, si en el manual dice bien claro que modo usar...
Cita:
Iniciado por http://www.php.net/manual/en/function.fopen.php 'x' Create and open for writing only; place the file pointer at the beginning of the file. If the file already exists, the fopen() call will fail by returning FALSE and generating an error of level E_WARNING. If the file does not exist, attempt to create it. This is equivalent to specifying O_EXCL|O_CREAT flags for the underlying open(2) system call.