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copied!<p>If you want to ensure you are working with "Array of Foo" and you want to ensure methods receive "Array of Foo", you can:</p> <pre><code>class ArrayOfFoo extends \ArrayObject { public function offsetSet($key, $val) { if ($val instanceof Foo) { return parent::offsetSet($key, $val); } throw new \InvalidArgumentException('Value must be a Foo'); } } </code></pre> <p>then:</p> <pre><code>function workWithFoo(ArrayOfFoo $foos) { foreach ($foos as $foo) { // etc. } } $foos = new ArrayOfFoos(); $foos[] = new Foo(); workWithFoo($foos); </code></pre> <p>The secret sauce is that you're defining a new "type" of "array of foo", then passing that "type" around using type hinting protection.</p> <hr> <p>The <a href="http://haldayne-docs.readthedocs.org/en/latest/boost/" rel="nofollow noreferrer">Haldayne library</a> handles the boilerplate for membership requirement checks if you don't want to roll your own:</p> <pre><code>class ArrayOfFoo extends \Haldayne\Boost\MapOfObjects { protected function allowed($value) { return $value instanceof Foo; } } </code></pre> <p><em>(Full-disclosure, I'm the author of Haldayne.)</em></p> <hr> <p>Historical note: the <a href="https://wiki.php.net/rfc/arrayof" rel="nofollow noreferrer">Array Of RFC</a> proposed this feature back in 2014. The RFC was declined with 4 yay and 16 nay. The concept recently <a href="https://marc.info/?l=php-internals&m=150896437718050&w=2" rel="nofollow noreferrer">reappeared on the internals list</a>, but the complaints have been much the same as levied against the original RFC: adding this check would <a href="http://rwec.co.uk/blog/2016/10/towards-a-consistent-php-type-checking-system/" rel="nofollow noreferrer">significantly effect performance</a>.</p>

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