Opcode Descriptions and Examples
PHP Manual

ASSIGN_SUB

PHP code

<?php
/*
 * Subtract value1 from value2 and store in variable indicated by value1
 * opcode number: 24
 */
$a-=3;
?>

PHP opcodes

Function name: (null)

Compiled variables: !0=$a

line#op fetchextreturn operands
60 ASSIGN_SUB     !0,3
71 RETURN     1

Opcode Descriptions and Examples
PHP Manual