What is the meaning of operation in c

I am trying to serialize PHP opcode. I am confused with code. Please check following code. What is the meaning of the code.

(ptr) = (void*)((char*)(ptr) - (char*)script->mem)

https://github.com/php/php-src/blob/6aaab9adf7619c121c19701022aeb8d88f9c3bab/ext/opcache/zend_file_cache.c#L112

How to serialize op_array ?