PHP: Example analysis of two syntax extensions of foreach

一foreach Syntax introduction

Versions of PHP 4 and above include the foreach structure, which is just a traversing array simple method. 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 traverses 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. And 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 use "@" to suppress error messages.

2 Summary of PHP foreach

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;
 
}

The result is: 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;
 
}

The result is: 0

This is because the $arr used in the loop points directly to the original data, rather than a copy.

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;
 
}

The result is: 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;
 
}

The result is: 0

2. If twice When looping an array, 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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