collection & php function parameter - bug?

I don't expect a php function to modify its parameters (not passed by reference)

Can you explain why es2 function below modify parameter c1 ?

	 function es1($p1) {
		$p1++;
		return $p1;
	}
	function es2($c1) {
		$c1->pull(0);
		$c1=$c1->values();
		return $c1;
	}

$p1=2;
echo "p1=$p1<br>";
$p2=$this->es1($p1);
echo "p1=$p1 p2=$p2<br>";

// p1=2
// p1=2 p2=3

$c1=collect([1,2]);
echo "c1=$c1<br>";
$c2=$this->es2($c1);
echo "c1=$c1 c2=$c2<br>";

// c1=[1,2]
// c1={"1":2} c2=[2]