PHP analysis on $i++ and ++$i

This article uses the VLD tool to analyze the opcode of php to explain the reason. First, post the picture

T1.php code

$i = 1;

$i+$i++;

Vld code

number of ops: 5

compiled vars: !0 = $i

line # * op fetch ext return operands

------------------------- -------------------------------------------------- --------

2 0 > ASSIGN                                                                                                                                 ~2                                                                                      gt ; RETURN                                                                                     4

path #1: 0,

T2.php code

$i = 1;

$i+$i+$i++

Vld2 code

number of ops: 6

compiled vars: !0 = $i

line # * op fetch ext return operands

--------- -------------------------------------------------- -----------------------

2 0 > ASSIGN                                                                                                                                                                                                                          

2 POST_INC 3 ADD                                                                                                ~3

4 5 > RETURN                                            2- 4; sop: 0 ; eop: 5

path #1: 0,

Let’s compare the two pictures 2 and 4, and you can know why the results will surprise you

First analyze the execution results of 1 and 2

First $ i is assigned a value of 1, $i++ is incremented, the result 1 is copied to the temporary variable ~1 ($i), and then $i is incremented to 2, that is! 0=2, of course the final result ~2 = ~1+! 0 ===3;

Let’s analyze 3,4

First, $i is assigned a value of 1, then $i+$i = !0+!0 == ~1 == 2, then $i++ is incremented, and The result 1 is copied to the temporary variable ~2 ($i), and the final result ~1+~2 == 2+1 ===3;