PHP - dudas con las llaves en php

<?php

class OBJ{

	private $array=array();

	public function __construct(){

		$this->array=array(

			1,2,3,

			"a","b",'c',NULL

		);

	}

	public function getArray(){

		return $this->array;

	}

	public function getId($i){

		return $this->array[$i];

	}

}

$tests=new OBJ();

//$tests=array(2,3,4,"a","asd",NULL);

foreach ($tests->getArray() as $key => $element) {

	if (is_numeric($element)){

		echo "'$element' is numeric", PHP_EOL;

	} else {

		echo "'{$tests->getId($key)}' is NOT numeric", PHP_EOL;

	}

}

?>