You might not know this about PHP!

iterator_iteration.php

<?php

/* Classes implementing iterators can not be used for nested iterating without ugly side effects.
 * Iterations over arrays create copies of the arrays with seperate array pointers.
 * Iterations over Iterater will only call the object which does not know in which loop it get's called and can not create a second or third pointer on the array for nested iterating.
 * To dodge this we must use the array. If the iterator implements a toArray method we're fine. If not, we still can use iterator_to_array().
 */

// Expected

$arr = [
    'alpha',
    'beta',
    'gamma',
];
echo '------------------' . PHP_EOL;
foreach ($arr as $char) {
    foreach ($arr as $char1) {
        echo $char1 . PHP_EOL;
    }
    echo $char . PHP_EOL;
}
echo '------------------' . PHP_EOL;

// OUTPUT:
/*
------------------
alpha
beta
gamma
alpha
alpha
beta
gamma
beta
alpha
beta
gamma
gamma
------------------
*/

// Actual

class Collection implements Iterator
{
    protected $arr = [
        'alpha',
        'beta',
        'gamma',
    ];

    public function current() { return current($this->arr); }
    public function next() { next($this->arr); }
    public function key() { return key($this->arr); }
    public function valid() { return key($this->arr) !== null; }
    public function rewind() { reset($this->arr); }
    public function toArray() { return $this->arr; }
}

$coll0 = new Collection();

echo '------------------' . PHP_EOL;
foreach ($coll0 as $char) {
    foreach ($coll0 as $char1) {
        echo $char1 . PHP_EOL;
    }
    echo $char . PHP_EOL;
}
echo '------------------' . PHP_EOL;

// OUTPUT
/*
------------------
alpha
beta
gamma
alpha
------------------
*/

/*
 * HOW TO DODGE
 */

// Possibility 1: Cloning
foreach (clone $collection as $char) {
    foreach (clone $collection as $char1) {
        echo $char1 . PHP_EOL;
    }
    echo $char . PHP_EOL;
}

// Possibility 2: Accessing the array
foreach ($collection->toArray() as $char) {
    foreach ($collection->toArray() as $char1) {
        echo $char1 . PHP_EOL;
    }
    echo $char . PHP_EOL;
}

// Possibility 3: Get an array copy with iterator_to_array
$array = iterator_to_array($collection);
foreach ($array as $char) {
    foreach ($array as $char1) {
        echo $char1 . PHP_EOL;
    }
    echo $char . PHP_EOL;
}

// OUTPUT of the possibilities above:
/*
------------------
alpha
beta
gamma
alpha
alpha
beta
gamma
beta
alpha
beta
gamma
gamma
------------------
*/