PHP循环问题：$a += 3; 在for循环中的运行机制解惑

Hey there! No question is too "shallow"—it's totally normal to get stuck on these basics until you walk through them step by step. Let's break down exactly what's happening in your code so the output of 15 makes perfect sense.

First, here's your test code for reference:

<?php 
$a = 0; 
for ($i=0; $i<5; $i++) { 
  $a += 3; // How this actually works 
} 
echo $a; // Echoing 15 
?>

What does $a += 3 do? #

$a += 3 is just a convenient shorthand for $a = $a + 3. Here's the play-by-play:

1. It grabs the current value stored in $a.
2. Adds 3 to that value.
3. Saves the new total back into $a.

Walking through the for loop #

Your for loop runs as long as $i < 5. Let's count the iterations: $i starts at 0, increments by 1 each loop, and stops when $i hits 5. That means the loop runs 5 total times (when $i is 0, 1, 2, 3, 4).

Let's track $a through each loop:

• Loop 1 ($i = 0): $a starts at 0 → 0 + 3 = 3 → $a becomes 3.
• Loop 2 ($i = 1): $a is 3 → 3 + 3 = 6 → $a becomes 6.
• Loop 3 ($i = 2): $a is 6 → 6 + 3 = 9 → $a becomes 9.
• Loop 4 ($i = 3): $a is 9 → 9 + 3 = 12 → $a becomes 12.
• Loop 5 ($i = 4): $a is 12 → 12 + 3 = 15 → $a becomes 15.

After the loop finishes, you echo $a—which is why you see 15 printed. To put it simply: 5 loops × 3 added per loop = 15.

内容的提问来源于stack exchange，提问作者kronberger