The "array_key_exists" function checks if the given key or index exists in the array.

    array_key_exists(key, array)

In the above syntax the "key" is checked for in the "array". The function returns "true" if the key is present, and "false" otherwise.

    <?php
    $a = array('apple' => 1, 'guava' => 4);
    if (array_key_exists('apple', $a)) 
      {
       echo "The Key 'apple' exists";
      }
    else
      {
       echo "The Key 'apple' does'nt exists";
      }
      }
    ?>

    The Key 'apple' exists

In the above example the array "$a", is checked for the key 'first', since present it displays the message.