Difference between "as $key => $value" and "as $value" in PHP foreach

Let's say you have an associative array like this:

$a = array(
    "one" => 1,
    "two" => 2,
    "three" => 3,
    "seventeen" => array('x'=>123)
);

In the first iteration : $key="one" and $value=1.

Sometimes you need this key ,if you want only the value , you can avoid using it.

In the last iteration : $key='seventeen' and $value = array('x'=>123) so to get value of the first element in this array value, you need a key, x in this case: $value['x'] =123.

A very important place where it is REQUIRED to use the key => value pair in foreach loop is to be mentioned. Suppose you would want to add a new/sub-element to an existing item (in another key) in the $features array. You should do the following:

foreach($features as $key => $feature) {
    $features[$key]['new_key'] = 'new value';  
} 

Instead of this:

foreach($features as $feature) {
    $feature['new_key'] = 'new value';  
} 

The big difference here is that, in the first case you are accessing the array's sub-value via the main array itself with a key to the element which is currently being pointed to by the array pointer.

While in the second (which doesn't work for this purpose) you are assigning the sub-value in the array to a temporary variable $feature which is unset after each loop iteration.

Well, the $key => $value in the foreach loop refers to the key-value pairs in associative arrays, where the key serves as the index to determine the value instead of a number like 0,1,2,... In PHP, associative arrays look like this:

$featured = array('key1' => 'value1', 'key2' => 'value2', etc.);

In the PHP code: $featured is the associative array being looped through, and as $key => $value means that each time the loop runs and selects a key-value pair from the array, it stores the key in the local $key variable to use inside the loop block and the value in the local $value variable. So for our example array above, the foreach loop would reach the first key-value pair, and if you specified as $key => $value, it would store 'key1' in the $key variable and 'value1' in the $value variable.

Since you don't use the $key variable inside your loop block, adding it or removing it doesn't change the output of the loop, but it's best to include the key-value pair to show that it's an associative array.

Also note that the as $key => $value designation is arbitrary. You could replace that with as $foo => $bar and it would work fine as long as you changed the variable references inside the loop block to the new variables, $foo and $bar. But making them $key and $value helps to keep track of what they mean.