function里面如何定义ref输出的参数

function fo($a,$b,$cheng,$jia)
{
  $cheng = $a*$b;
  $jia = $a + $b;
}

function fo($a, $b, &$cheng, &$jia)
{
  $cheng = $a*$b;
  $jia = $a + $b;
}

function fo($a,$b,&$cheng,&$jia)
{
  $cheng = $a*$b;
  $jia = $a + $b;
}

fo(1,2,$cheng, $jia);

echo $cheng; // 2
echo $jia;   // 3