Fixed E_NOTICE in listing worker queue and new utilities class added (#5521)

* Fixes for E_NOTICE in workqueue:
- introduced class `Friendica\Util\Arrays` which will hold static methods for
  handling arrays that cannot be done with PHP's functions, like implode() on
  multi-dimensional arrays
- rewrote old-school for() loop to foreach()

* Added intial unit test with some tests on empty delimiters and/or sinle and
multi-dim array.

* Added test for for 3-dimensional arrays, thanks to  nupplaphil's feedback.

src/Util/Arrays.php

@@ -0,0 +1,49 @@
+<?php
+/**
+ * @file src/Util/Arrays.php
+ * @author Roland Haeder<https://f.haeder.net/profile/roland>
+ */
+namespace Friendica\Util;
+
+/**
+ * @brief Array utility class
+ */
+class Arrays
+{
+	/**
+	 * @brief Private constructor
+	 */
+	private function __construct () {
+		// Utitlities don't have instances
+	}
+
+	/**
+	 * @briefs Implodes recursively a multi-dimensional array where a normal implode() will fail.
+	 *
+	 * @param array  $array Array to implode
+	 * @param string $glue  Glue for imploded elements
+	 * @return string String with elements from array
+	 */
+	public static function recursiveImplode (array $array, $glue) {
+		// Init returned string
+		$string = '';
+
+		// Loop through all records
+		foreach ($array as $element) {
+			// Is an array found?
+			if (is_array($element)) {
+				// Invoke cursively
+				$string .= '{' . self::recursiveImplode($element, $glue) . '}' . $glue;
+			} else {
+				// Append normally
+				$string .= $element . $glue;
+			}
+		}
+
+		// Remove last glue
+		$string = trim($string, $glue);
+
+		// Return it
+		return $string;
+	}
+}