ESP8266 PHP Web Server
It sounds like you are trying to put a PHP file on an SD card that the ESP8266 will serve itself.

This is not possible, as no one has, and probably never will, port PHP to the Espressif 8266 platform.

Your device can serve .htm .css and .js files because it's just a matter of streaming the file content to the client. But PHP requires that an application on the server run the PHP code, and then send the resulting output to the client.