PHP code not through OOP

Extending from a parent class allows the child to inherit all attributes and abilities of the parent. Step 1: Add a new method to the Developer class named displayUserSkills. Step 2: Use the 'name' from the PARENT class, and the 'skills' from the CURRENT class, to replace the <PLACEHOLDERS> and RETURN following string (NOTE that skills separated with a comma and space): <NAME> has the following skills: <SKILL1>, <SKILL2>, <SKILL3> Example Output: Alena has the following skills: PHP, MySQL, HTML

my code so far:

<?php

//add new child class here class Developer extends User { protected $skills = array();

public function getSkills(){ return $this->skills; }

public function setSkills($an_array){ $this->skills = $an_array; }

public function displayUserSkills(){ return $this -> getSkills();

}

}

$an_array = array("PHP","MySQL","HTML");

$stuff = new Developer ;

$stuff -> setSkills($an_array);

$stuff -> setName('Alena');

echo $stuff->getName() . " has the following skills: " . implode(', ', $stuff->displayUserSkills());

?>

Out put error;

Alena has the following skills: PHP, MySQL, HTMLPHP Warning: strtolower() expects parameter 1 to be string, array given in 37235294-194c-4d9b-943d-c20e202f54c4.php on line 224 Warning: strtolower() expects parameter 1 to be string, array given in 37235294-194c-4d9b-943d-c20e202f54c4.php on line 224

anyone knows why?