[slow_typist]

Humans have to put the so called php-file on the server intentionally for any subsequent attack to work. But it is a binary file.

[h33t-l4x0r]

I imagine it's supposed to get onto the server by an exploited vulnerable image upload plugin

[slow_typist]

Maybe I don’t understand the scenario fully, but under your assumption there is no need to inject the malicious webshell later.