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<?php $array = [ "foo" => "no", "foo_contact_firstname" => "Mark", "foo_email" => "mark@gmail.com", "foo_address1" => "91", "foo_address2" => "old road", "foo_bill" => "no", "bar" => "yes", "bar_contact_lastname" => null, "bar_telephone" => null, "bar_email" => null, "bar_address1" => null, "bar_address2" => null, "bar_address3" => null, "bar_address4" => null, "bar_address5" => null, "bar_postcode" => null, "register_contact_firstname" => null, "register_contact_lastname" => null, "register_telephone" => null, "register_email" => null, "register_address1" => null, "register_address2" => null, "register_address3" => null, "register_address4" => null, "register_address5" => null, "register_postcode" => null, ]; $test = array_filter($array, function($key) { return $key == 'foo'; }, ARRAY_FILTER_USE_KEY ); var_dump($test);
Output for 5.6.38 - 7.3.0rc2
array(1) { ["foo"]=> string(2) "no" }