Debug Impossible PHP Program Bug

The 3 attached JPEGs prove that the program on my screen is the one being executed. When I add additional blank lines and run the program the error message increases the line number accordingly. However, the program itself is impossible as it executes echo and exit commands near the top but it continues to execute and get the same error message, while increasing the line number accordingly, as well as $f being defined contrary to the error message. Before we talk, please tell me if you (can) see the problem and tell me the next prime number after 7. Charlie