Two uses of PHP foreach as $key => $value

This article introduces the two usages of PHP foreach as $key => $value, which has a certain reference value. Now I share it with you. Friends in need can refer to it

1. Introduction to the syntax of foreach

PHP 4 and above versions include the foreach structure, which is just a convenient way to traverse an array. foreach can only be used with arrays, and an error will occur when trying to use it with other data types or an uninitialized variable. There are two syntaxes, the second is a useful extension of the first.

foreach(array_expression as $value) statement
foreach(array_expression as $key => $value) statement

The first format iterates over the given array_expression array. Each time through the loop, the value of the current cell is assigned to $value and the pointer inside the array is moved forward one step (so the next cell will be obtained in the next loop).

The second format does the same thing, except that in addition to the value of the current cell, the key value will also be assigned to the variable $key in each loop. Look at the following code:

$arr = array("1"=>"111","2"=>"222","3"=>"333");
foreach($arr as $key=>$value)
{
echo $key."=>".$value."\n";
}

结果如下：
1=>111
2=>222
3=>333

The key value here can be understood as the array subscript. The subscript of the array element a[2] is 2.

When foreach starts to execute, the pointer inside the array will Automatically points to the first unit. This means there is no need to call reset() before the foreach loop. The while loop requires reset. The two codes below function exactly the same.

1. Use while loop

$arr = array("one", "two", "three");
reset ($arr);
while (list(, $value) = each ($arr)) {
echo "Value: $value<br>\n";
}

2. Use foreach

foreach ($arr as $value) {
echo "Value: $value<br>\n";
}

Also note that foreach operates a copy of the specified array, not the array itself . Therefore, even with the construction of each(), the original array pointer has not changed, and the value of the array unit has not been affected.

foreach does not support the ability to suppress error messages using "@".

2. PHP foreach summary

1. The use of references in foreach.

Under normal circumstances, $arr and $value in foreach($arr as $value) are copies and are not affected by external factors, that is,

$arr = array(0,1,2,3,4,5);
foreach($arr as $value){
$arr = array();
echo $value;
}

结果是：12345

But if $arr is a reference, The situation is different. We use code to illustrate the problem

$arr = array(0,1,2,3,4,5);
$arr = &$arr;
foreach($arr as $value){
$arr = array();
echo $value;
}

结果是: 0

This is because the $arr used in the loop points directly to the original data instead of copying it.

If $value is a reference and $arr is not a reference, the result is the same. Similarly, $value points to the original data instead of copy

$arr = array(0,1,2,3,4,5);
foreach($arr as &$value){
$arr = array();
echo $value;
}

结果是: 0

There is another special situation, that is If $arr is defined as a global variable, $arr will also become a reference

global $arr;
$arr = array(0,1,2,3,4,5);
foreach($arr as $value){
$arr = array();
echo $value;
}

结果是: 0

2. If you loop an array twice, you must not write it like this

foreach($arr as &$value){}
foreach($arr as $value){}

This will cause the second The result of the loop is incorrect (maybe it is a PHP bug). You can use the following instead:

//solution 1
foreach($arr as &$value){}
unset($value);
foreach($arr as $value){}
//solution 2
foreach($arr as &$value){}
foreach($arr as &$value){}
//solution 3
foreach($arr as &$value){}
$arr2 = $arr;
foreach($arr2 as $value){}

Related recommendations:

Detailed explanation of the usage and examples of foreach in PHP

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