<?php
$search_array = array('first' => 1, 'second' => 4);
if (array_key_exists('first', $search_array)) {
echo "The 'first' element is in the array";
}
?>
//your array
$arr1 = array (
"y" => 35,
"x" => 51,
"z" => 35,
"c_3" => 4,
"c_1" => 54,
"c_6" => 53,
"c_9" => 52
);
// Array with keys you want
$arr2 = array (
"c_3" => '',
"c_1" => '',
"c_6" => '',
"c_9" => ''
);
//use array_intersect_key to find the common ;)
print_r(array_intersect_key($arr1,$arr2));