source: vendor/google/apiclient/src/Collection.php

<?php

namespace Google;

/**
 * Extension to the regular Google\Model that automatically
 * exposes the items array for iteration, so you can just
 * iterate over the object rather than a reference inside.
 */
class Collection extends Model implements \Iterator, \Countable
{
    protected $collection_key = 'items';

    /** @return void */
    #[\ReturnTypeWillChange]
    public function rewind()
    {
        if (
            isset($this->{$this->collection_key})
            && is_array($this->{$this->collection_key})
        ) {
            reset($this->{$this->collection_key});
        }
    }

    /** @return mixed */
    #[\ReturnTypeWillChange]
    public function current()
    {
        $this->coerceType($this->key());
        if (is_array($this->{$this->collection_key})) {
            return current($this->{$this->collection_key});
        }
    }

    /** @return mixed */
    #[\ReturnTypeWillChange]
    public function key()
    {
        if (
            isset($this->{$this->collection_key})
            && is_array($this->{$this->collection_key})
        ) {
            return key($this->{$this->collection_key});
        }
    }

    /** @return mixed */
    #[\ReturnTypeWillChange]
    public function next()
    {
        return next($this->{$this->collection_key});
    }

    /** @return bool */
    #[\ReturnTypeWillChange]
    public function valid()
    {
        $key = $this->key();
        return $key !== null && $key !== false;
    }

    /** @return int */
    #[\ReturnTypeWillChange]
    public function count()
    {
        if (!isset($this->{$this->collection_key})) {
            return 0;
        }
        return count($this->{$this->collection_key});
    }

    /** @return bool */
    #[\ReturnTypeWillChange]
    public function offsetExists($offset)
    {
        if (!is_numeric($offset)) {
            return parent::offsetExists($offset);
        }
        return isset($this->{$this->collection_key}[$offset]);
    }

    /** @return mixed */
    #[\ReturnTypeWillChange]
    public function offsetGet($offset)
    {
        if (!is_numeric($offset)) {
            return parent::offsetGet($offset);
        }
        $this->coerceType($offset);
        return $this->{$this->collection_key}[$offset];
    }

    /** @return void */
    #[\ReturnTypeWillChange]
    public function offsetSet($offset, $value)
    {
        if (!is_numeric($offset)) {
            parent::offsetSet($offset, $value);
        }
        $this->{$this->collection_key}[$offset] = $value;
    }

    /** @return void */
    #[\ReturnTypeWillChange]
    public function offsetUnset($offset)
    {
        if (!is_numeric($offset)) {
            parent::offsetUnset($offset);
        }
        unset($this->{$this->collection_key}[$offset]);
    }

    private function coerceType($offset)
    {
        $keyType = $this->keyType($this->collection_key);
        if ($keyType && !is_object($this->{$this->collection_key}[$offset])) {
            $this->{$this->collection_key}[$offset] =
                new $keyType($this->{$this->collection_key}[$offset]);
        }
    }
}