<?php
error_reporting(-1);

$balls = [
			["color" => "green", "cost" => 1, "weight" => 5],
			["color" => "red", "cost" => 3, "weight" => 3],
			["color" => "blue", "cost" => 5, "weight" => 2]
		];
	

function grabBalls($balls, $ballsTaken) {
	foreach($balls as $ball) {
		$ballsTaken[] = $ball["color"];
		
		if (count($ballsTaken) < 5) {
			grabBalls($balls, $ballsTaken);
		} else {
			var_dump($ballsTaken);
		
		}
		
	return $ballsTaken;	
	}
}

$a = [];	
grabBalls($balls, $a);

var_dump($a);