Some problems with PHP foreach array loop_PHP tutorial

PHP foreach() syntax structure is used to traverse operations or output arrays. foreach() can only be used to traverse arrays or objects. It will occur when trying to use it for other data types or an uninitialized variable. mistake.

Syntax:

foreach (array as $value)
    statement
	// 或者：
foreach (array as $key => $value)
    statement

In the above syntax, each loop assigns the value of the current unit to $value and moves the pointer inside the array forward one step. In the second syntax format, the key name of the current unit is also assigned to the variable $key in each loop.

Usually a for loop is used to access arrays, for example:

for($i = 0; $i < 3; $i++)
{
	echo $arr[$i];
}

But for operating a large array through manual code, using a foreach loop requires less code. The above code can be written like this:

foreach ($arr as $value)
{
	echo $value;
}

Let’s discuss some issues in using foreach in detail.

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;
}

// 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;
}

// 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 through an array twice, you must not write it like this

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

This will cause the result of the second loop to be 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){}

3. To prevent foreach from being undefined, try to write foreach like this

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

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