Thats so stupid. A single PHP File is a bad idea.
If all Functions are in a single file, it means: It always loads the PHP on the server, no matter what function you really need and the File must be huge. Its just a load to do for the server even if its a simple function.
Also, everyone can say its a single php. You just use either require or include inside the index.php and say its a single PHP.
I guess no dev will fall for this. Its just stupid to say so and shows the guy is probably not a Developer at all....