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.

tests/src/Util/ArraysTest.php

@@ -0,0 +1,114 @@
+<?php
+/**
+ * @file tests/src/Util/Arrays.php
+ * @author Roland Haeder<https://f.haeder.net/profile/roland>
+ */
+namespace Friendica\Test\Util;
+
+use Friendica\Util\Arrays;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @brief Array utility testing class
+ */
+class ArraysTest extends TestCase
+{
+	/**
+	 * @brief Tests if an empty array and an empty delimiter returns an empty string.
+	 */
+	public function testEmptyArrayEmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([], '');
+		$this->assertEmpty($str);
+	}
+
+	/**
+	 * @brief Tests if an empty array and a non-empty delimiter returns an empty string.
+	 */
+	public function testEmptyArrayNonEmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([], ',');
+		$this->assertEmpty($str);
+	}
+
+	/**
+	 * @brief Tests if a non-empty array and an empty delimiter returns the value (1).
+	 */
+	public function testNonEmptyArrayEmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([1], '');
+		$this->assertSame($str, '1');
+	}
+
+	/**
+	 * @brief Tests if a non-empty array and an empty delimiter returns the value (12).
+	 */
+	public function testNonEmptyArray2EmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([1, 2], '');
+		$this->assertSame($str, '12');
+	}
+
+	/**
+	 * @brief Tests if a non-empty array and a non-empty delimiter returns the value (1).
+	 */
+	public function testNonEmptyArrayNonEmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([1], ',');
+		$this->assertSame($str, '1');
+	}
+
+	/**
+	 * @brief Tests if a non-empty array and a non-empty delimiter returns the value (1,2).
+	 */
+	public function testNonEmptyArray2NonEmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([1, 2], ',');
+		$this->assertSame($str, '1,2');
+	}
+
+	/**
+	 * @brief Tests if a 2-dim array and an empty delimiter returns the expected string.
+	 */
+	public function testEmptyMultiArray2EmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([[1], []], '');
+		$this->assertSame($str, '{1}{}');
+	}
+
+	/**
+	 * @brief Tests if a 2-dim array and an empty delimiter returns the expected string.
+	 */
+	public function testEmptyMulti2Array2EmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([[1], [2]], '');
+		$this->assertSame($str, '{1}{2}');
+	}
+
+	/**
+	 * @brief Tests if a 2-dim array and a non-empty delimiter returns the expected string.
+	 */
+	public function testEmptyMultiArray2NonEmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([[1], []], ',');
+		$this->assertSame($str, '{1},{}');
+	}
+
+	/**
+	 * @brief Tests if a 2-dim array and a non-empty delimiter returns the expected string.
+	 */
+	public function testEmptyMulti2Array2NonEmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([[1], [2]], ',');
+		$this->assertSame($str, '{1},{2}');
+	}
+
+	/**
+	 * @brief Tests if a 3-dim array and a non-empty delimiter returns the expected string.
+	 */
+	public function testEmptyMulti3Array2NonEmptyDelimiter()
+	{
+		$str = Arrays::recursiveImplode([[1], [2, [3]]], ',');
+		$this->assertSame($str, '{1},{2,{3}}');
+	}
+}