Flattens a multidimensionnal array in PHP mashing the keys together using a parametized separator.

flatten_array.php

/**
 * Flatten a multi dimension array. For an associative array the child keys are separated by underscore(_) (overwrittable).
 *
 * Function taken and tweaked from https://gist.github.com/mmohiudd/7474896
 * @author Muntasir Mohiuddin
 * @param array $a multidimensional array to flatten
 * @param array &$r multidimensional array to save values
 * @param string $s separator to use between parent and children keys
 * @param string $p previous parents separated by the separator
 * @return array all flattened keys
 */
public static function flatten_array($a, &$r, $s = "_", $p = NULL){
	$keys = [];

	foreach($a as $k => $v) {
		$keys[] = $k;

		if(is_null($p)){
			$_p = $k;
		} else {
			$_p = $p . $s . $k;
		}

		if (is_array($a[$k])) {
			$keys = array_merge($keys, self::flatten_array($a[$k], $r, $s, $_p));
		} else {
			$r[$_p] = $v;
		}
	}

	return $keys;
}