Understanding PHP Execution: Zend VM Process, Opcodes, and Opcache

The article begins by outlining the four compilation stages of a C program—pre‑processing, compilation, assembly, and linking—and maps these concepts to PHP, where source code is first tokenised, then parsed into an abstract syntax tree (AST) before being compiled into an op_array of opcodes.

It then describes the PHP implementation, explaining that the Zend VM (ZendVM) interprets the op_array using a set of handlers. Each opcode (opline) is defined by a zend_op structure, and the possible operand types are enumerated as IS_CONST , IS_TMP_VAR , IS_VAR , IS_UNUSED , and IS_CV . Example handler code for the ADD opcode shows how different operand type combinations select different specialised functions.

The execution process is broken into four steps: (1) creating a new zend_execute_data frame with zend_vm_stack_push_call_frame , (2) initializing the frame via i_init_code_execute_data which attaches the symbol table and loads runtime caches, (3) running a loop that dispatches each opcode using one of four dispatch modes (CALL, SWITCH, GOTO, HYBRID) – the default being CALL – and handling the four possible return codes ( ZEND_VM_ENTER , ZEND_VM_LEAVE , ZEND_VM_RETURN , ZEND_VM_CONTINUE ), and (4) freeing the call frame, which is actually cached for reuse.

Finally, the article discusses Opcache, the shared‑memory extension that stores compiled zend_persistent_script structures to avoid recompilation on each request. It outlines the structure of the cached script, the initialization of the extension via accel_startup , and the workflow of retrieving or generating the cache.