WHY do PHP reindex arrays and add integer indexes?

ALL PHP ARRAYS are associative arrays.

So your simple single dimensional array of : 

PHP Code:

$myArray = ['apple', 'banana', 'cherry']; 


is structurally the same as: 

PHP Code:

$yourArray = [0 => 'apple', 1 => 'banana', 2 => 'cherry']; 


So, if you will using foreach like this:

PHP Code:

foreach ($myArray as $index => $value)
{
    echo $index . ' ' . $value;
}

// You'll get
// 0 apple
// 1 banana
// 2 cherry 


That's simply how PHP internals work. If you ask why is that, then you need to ask the PHP maintainers themselves.