A brief discussion on the pitfalls of using reference variables in foreach in PHP

I have been writing PHP for many years, but I still make low-level mistakes. Today I encountered a pitfall when referencing variables in foreach. The PHP version is 5.6.12. The code is as follows:

<?php
$arr = [&#39;a&#39;, &#39;b&#39;, &#39;c&#39;, &#39;d&#39;, &#39;e&#39;];
foreach ($arr as $i=>&$a) {
  $a = $a.&#39;_&#39;. $a;
  echo $a .&#39;<br>&#39;;
}
echo &#39;<hr>&#39;;
foreach ($arr as $i=>$a) {
  echo $a .&#39;<br>&#39;;
}
echo &#39;<hr>&#39;;
print_r($arr);

Output results

When I first saw the output result of the second foreach, I felt very confused. How could two d_d be output?

After thinking about it carefully, it turns out that the scope of $a in PHP foreach is a local variable of the entire function, which is still valid outside the loop, rather than being enclosed within the loop,

So when the second foreach is executed, $a is not a new variable, but still points to the address reference of the fifth element of the $arr array.

When the second foreach is looping, it is actually in Constantly assign values ​​to the 5th element of the $arr array,

Specific assignment situation,

First time: a_a is assigned to the 5th element, the result is: [a_a, b_b, c_c, d_d,a_a]

The second time: b_b is assigned to the fifth element, the result is: [a_a, b_b, c_c, d_d,b_b]

The third time: c_c is assigned Assign the value to the 5th element, the result is: [a_a, b_b, c_c, d_d,c_c]

The fourth time: d_d is assigned to the 5th element, the result is: [a_a, b_b, c_c, d_d, d_d]

The fifth time: At this time, because the fifth element has become d_d, d_d is assigned to the fifth element again, and the result is still: [a_a, b_b, c_c, d_d,d_d]

Solution:

1. Try not to use the same loop variable name;

2. Perform unset($a); after each use or before using it again. To cancel the address application

still use the above code example:

$arr = ['a', 'b', 'c', 'd', 'e'];
foreach ($arr as $i=>&$a) {
  $a = $a.'_'. $a;
  echo $a .&#39;<br>&#39;;
}
echo &#39;<hr>&#39;;
echo $a;
echo &#39;<hr>&#39;;
 
// 这里 unset 掉
unset($a);
 
echo $a = &#39;ccc&#39;;
echo &#39;<hr>&#39;;
print_r($arr);
echo &#39;<hr>&#39;;
foreach ($arr as $i=>$a) {
  echo $a .&#39;<br>&#39;;
}
echo &#39;<hr>&#39;;
print_r($arr);

Output result:

Now it is normal, these small details must be Attention

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