PHP foreach loop example confusion

Question

One of the examples of a foreach loop in PHP is

foreach ($row as $key => $value) { 
    // somecode
}

I am having trouble understanding what the => is doing. Can anyone run me through how it is "seen" or evaluated by PHP? What is the order of operation, what value is assigned to $key?

What makes it different from:

foreach ($row as $value) { 
    // somecode
}

?

I logically I thought that the value of $value would be assigned to $key, then it would be assigned as a row of $row, but that is obviously incorrect...

Answer 1

I understand it is a little tricky, I had problems understanding it when I first started using it. The more you use it, the more it makes more sense.

Your array would look like this:

//  "key"   "value"
//    |        |
//    V        V
$row['1'] = 'item1';
$row['2'] = 'item2';
$row['3'] = 'item3';
$row['4'] = 'item4';

Keys can be anything, they don't need to be numbers. That way you can iterate through all items in the array without needing to know the key!

So, your first example can be explained as follows:

//         +--- The ARRAY where all of your data is
//         |
//         |       +----- The KEY to access that element of the array
//         |       |
//         |       |        +----- The VALUE of that element
//         |       |        |
//         V       V        V
foreach ($row as $key => $value){
   if($row[$key] == $value){ // this statement is always true
      echo "true AGAIN!";  // and thus will always print this line
   }
}

As far as my understanding goes the => is not really an operand of sorts, it is just used to complete the structure of the foreach loop.

Answer 2

PHP arrays are stored as pairs of keys and values:

$arr = array(
    'key1' => 'value1',
    'key2' => 'value2'
);

The standard foreach syntax gets just the values:

foreach ($arr as $value) {
    // value1, value2
}

The alternative syntax allows you to get keys as well:

foreach ($arr as $key => $value) {
    // $key is "key1", then "key2"
    // $value is "value1" then "value2"
}

See the manual entry for foreach.