What are the difference between array_keys() and array_key_exists() in php?

array_key_exists() : It is used to checks an array for a particular key and returns TRUE if the key exists and FALSE if the key does not exist.

array_keys() : This function returns an array containing the keys. It takes three parameters out of which one is mandatory and other two are optional.
Syntax : array_keys(array,value,strict)