array-key refers to key portion of array holding key and value pair. In PHP array_keys() returns the keys, numeric and string, from the input array.
array-key() refers to key portion of array holding key and value pair. In PHP array_keys() returns the keys, numeric and string, from the input array. 
array array_keys ( array $array [, mixed $search_value [, bool $strict = FALSE ]] )
array_keys()— Return all the keys or a subset of the keys of an array
Example:
$array = array(0 => 100, "color" => "red");
print_r(array_keys($array));
$array = array("blue", "red", "green", "blue", "blue");
print_r(array_keys($array, "blue"));
Result:
Array
(
    [0] => 0
    [1] => color
)
Array
(
    [0] => 0
    [1] => 3
    [2] => 4
)
 
     
     
     
     
     
     
     
     
     
     
     
     
     
    