Opcodes
PHP Manual

NOP

PHP code

<?php
/*
 * no operation
 * opcode number: 0
 */
function A(){}; 
?>

PHP opcodes

Function name: (null)

Compiled variables: none

line#op fetchextreturn operands
60 NOP      
71 RETURN     1

Function name: A

Compiled variables: none

line#op fetchextreturn operands
60 RETURN     null

Opcodes
PHP Manual