How to get array key from corresponding array value?

You can use the array_keys function for that.

Example:

$array = array("blue", "red", "green", "blue", "blue");
print_r(array_keys($array, "blue"));

This will get the key from the array for value blue

You could use array_search() to find the first matching key.

From the manual:

$array = array(0 => 'blue', 1 => 'red', 2 => 'green', 3 => 'red');

$key = array_search('green', $array); // $key = 2;
$key = array_search('red', $array);   // $key = 1;