Opcode Descriptions and Examples
PHP Manual

RECV_INIT

PHP code

<?php
/*
 * Initialize a function argument with "value" if not received from caller.  Otherwise same as RECV.
 * opcode number: 64
 */
function hello($a=5){}
?>

PHP opcodes

Function name: (null)

Compiled variables: none

line#op fetchextreturn operands
60 NOP      
71 RETURN     1

Function name: hello

Compiled variables: !0=$a

line#op fetchextreturn operands
60 RECV_INIT     1,5
 1 RETURN     null