2つの配列の差分、共通項を求めるには

差分を求めるにはarray_diff()

$fruit = array("apple","banana","grape","remon");
$juice = array("apple","orange","grape");

//配列juiceに存在しない、配列fruitの値を返す
$diff = array_diff($fruit, $juice);

//差分
while(list($key, $val) = each($diff)){
 echo $val . "\t";
}
//banana remon

共通項を求めるにはarray_intersect()

$ary1 = array("1"=>"apple","2"=>"banana","3"=>"grape");
$ary2 = array("a"=>"apple","b"=>"banana","c"=>"grape");
$unique = array_intersect($ary1, $ary2);
print_r($unique); //Array ( [1] => apple [2] => banana [3] => grape )