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

高洛峰
高洛峰Original
2017-01-03 14:42:101292browse

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

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

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:

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

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

Now it is normal, these small details must be Attention

The above is the brief discussion brought by the editor about the pitfalls of using reference variables in foreach in PHP. I hope everyone will support the PHP Chinese website~

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