Online PHP editor | output for sY2oD

<?php namespace Acme { class Collection { private $_items; public function __construct(array $items = [])/*: void*/ { $this->_items = $items; } public function filter(callable $callback): self { foreach ($this->_items as $key => $value) { if (!call_user_func($callback, $value)) { unset($this->_items[$key]); } } return $this; } public function map(callable $callback): self { foreach ($this->_items as $key => $value) { $this->_items[$key] = call_user_func($callback, $key, $value); } return $this; } public function reindex(): self { $this->_items = array_values($this->_items); } public function sprintf(string $format): self { return $this->map(function ($key, $value) use ($format) { return substr_count($format, '%s') === 1 ? sprintf($format, $value) : sprintf($format, $key, $value); }); } public function implode(string $delimiter = ''): string { $output = ''; $first = TRUE; foreach ($this->_items as $value) { if ($first) { $output .= $value; $first = FALSE; } else { $output .= "$delimiter$value"; } } return $output; } public static function property(string $property): callable { return function ($key, $value) use ($name) { return $value->$property; }; } } } namespace { use Acme\Collection; echo (new Collection([ 'one', 'two', (object)['foo' => 'FOO', 'bar' => 'BAR', 'baz' => 'BAZ'], 'three', (object)['bar' => 'BAR', 'qux' => 'QUX'], ])) ->filter('is_object') ->map(Collection::property('bar')) ->reindex() ->sprintf('(%s,%s)') ->implode('_');