PHP中$a++与++$a的自增自减规则解析及复杂表达式计算结果22的成因咨询

Hey there! Let's clear up how ++$a (prefix increment) and $a++ (postfix increment) work in PHP, then walk through your expression step by step to figure out where the result comes from.

Basic Rules #

First, let's lock down the core behavior:

• Postfix increment/decrement ($a++, $a--): Returns the current value of the variable first, then increments/decrements the variable by 1.
• Prefix increment/decrement (++$a, --$a): Increments/decrements the variable by 1 first, then returns the updated value.
• Unary negative (-$a) simply returns the negative of the variable's current value, with lower precedence than postfix increment/decrement but equal to prefix increment/decrement (right-associative).

Step-by-Step Breakdown of Your Expression #

You defined $a = 5, then calculated:

$a++ + $a-- + -$a + ++$a + $a++ + ++$a - $a-- + $a

Let's compute each term left to right, tracking $a's value and the cumulative result at every step:

1. First term: $a++

   • Postfix increment: Return current value 5, then $a becomes 6.
   • Cumulative result: 5

2. Second term: $a--

   • Postfix decrement: Return current value 6, then $a becomes 5.
   • Cumulative result: 5 + 6 = 11

3. Third term: -$a

   • Unary negative: Current $a is 5, so return -5.
   • Cumulative result: 11 + (-5) = 6

4. Fourth term: ++$a

   • Prefix increment: First set $a from 5 to 6, then return 6.
   • Cumulative result: 6 + 6 = 12

5. Fifth term: $a++

   • Postfix increment: Return current value 6, then $a becomes 7.
   • Cumulative result: 12 + 6 = 18

6. Sixth term: ++$a

   • Prefix increment: First set $a from 7 to 8, then return 8.
   • Cumulative result: 18 + 8 = 26

7. Seventh term: - $a--

   • Postfix decrement has higher precedence than unary negative: First return current $a value 8, then $a becomes 7, then apply the negative sign to get -8.
   • Cumulative result: 26 + (-8) = 18

8. Eighth term: $a

   • Current $a is 7, return 7.
   • Final result: 18 + 7 = 25

Wait a minute—this gives us 25, but you mentioned getting 22 when running the code. A few possible reasons for this discrepancy:

• Did you possibly mistype the expression (e.g., using --$a instead of - $a, or missing a term)?
• Was the initial value of $a not 5?
• Could there be other code modifying $a before this expression runs?

If you double-check the code with the exact expression you shared, it should output 25 in both PHP 7 and PHP 8.

内容的提问来源于stack exchange，提问作者Phạm Văn Hậu