2.2 Instructions

The instruction codes are defined in zend_vm_opcodes.h. Operators are converted to corresponding instruction codes when PHP scripts are compiled.

For example, the following assembly-like code represents $c = $a + $b (You can try that yourself using phpdbg):

ADD    $a, $b, ~0  # "+" operator 
ASSIGN $c, ~0      # "=" operator

However, not all operators have a corresponding instruction (e.g. negation, greater than, not less than operators). For example, PHP code $c = $a > -$b is compiled to something like:

MUL        $b, -1, ~0  # Converted to "$b * (-1)" (since PHP 7.3).
IS_SMALLER ~0, $a, ~1  # Converted to "<".
ASSIGN     $c, ~1