The PHP function presented in this page can be used to create Multi-Level Menu. The function receives three arguments:
      - $parentId - the ID of the parent /category with sub-categories.
      - $ctgLists - a two dimensional array with parents and their direct child-lists (their IDs).
      - $ctgData - a two dimensional array with data (name, link-URL) of each list /anchor in menu.

Code of the multilevelMenu() function

// recursive function to create multilevel menu list, $parentId 0 is the Root
// function addapted from: http://crisp.tweakblogs.net/blog/317/formatting-a-multi-level-menu-using-only-one-query.html
function multilevelMenu($parentId, $ctgLists, $ctgData) {
  $html = '';       // stores and returns the html code with Menu lists

  // if parent item with child IDs in ctgLists
  if(isset($ctgLists[$parentId])) {
    $html = '<ul>';      // open UL

    // traverses the array with child IDs of current parent, and adds them in LI tags, with their data from $ctgData
    foreach ($ctgLists[$parentId] as $childId) {
      // define CSS class in anchors, useful to be used in CSS style to design the menu
      if($parentId == 0) $clsa = ' class="firsrli"';       // class for anchors in main /first categories
      else if(isset($ctgLists[$childId])) $clsa = ' class="litems"';       // class for anchors in lists with childs
      else $clsa = '';

      // open LI
      $html .= '<li><a href="'. $ctgData[$childId]['lurl'] .'" title="'. $ctgData[$childId]['lname'] .'"'. $clsa .'>'. $ctgData[$childId]['lname'] .'</a>';

      $html .= multilevelMenu($childId, $ctgLists, $ctgData);     // re-calls the function to find parent with child-items recursively

      $html .= '</li>';      // close LI
    }
    $html .= '</ul>';       // close UL
  }

  return $html;
}

- Example of usage multilevelMenu() function:
<?php
// recursive function to create multilevel menu list, $parentId 0 is the Root
function multilevelMenu($parentId, $ctgLists, $ctgData) {
  $html = '';       // stores and returns the html code with Menu lists

  // if parent item with child IDs in ctgLists
  if(isset($ctgLists[$parentId])) {
    $html = '<ul>';      // open UL

    // traverses the array with child IDs of current parent, and adds them in LI tags, with their data from $ctgData
    foreach ($ctgLists[$parentId] as $childId) {
      // define CSS class in anchors, useful to be used in CSS style to design the menu
      if($parentId == 0) $clsa = ' class="firsrli"';       // class for anchors in main /first categories
      else if(isset($ctgLists[$childId])) $clsa = ' class="litems"';       // class for anchors in lists with childs
      else $clsa = '';

      // open LI
      $html .= '<li><a href="'. $ctgData[$childId]['lurl'] .'" title="'. $ctgData[$childId]['lname'] .'"'. $clsa .'>'. $ctgData[$childId]['lname'] .'</a>';

      $html .= multilevelMenu($childId, $ctgLists, $ctgData);     // re-calls the function to find parent with child-items recursively

      $html .= '</li>';      // close LI
    }
    $html .= '</ul>';       // close UL
  }

  return $html;
}

// array with parents and their childs
$ctgLists = array (
  0 => array (1, 33),
  1 => array (2, 15, 28, 31, 32),
  2 => array (3, 7, 12),
  3 => array (4, 5, 6),
  7 => array (8, 10, 11),
  8 => array (9),
  12 => array (13, 14),
  15 => array (16, 21, 22, 23, 27),
  16 => array (17, 18, 19, 20),
  23 => array (24, 25, 26),
  28 => array (29, 30),
  33 => array (34),
  34 => array (35, 38, 46),
  35 => array (36, 37),
  38 => array (39, 43),
  39 => array (40, 41, 42),
  43 => array (44, 45)
);

// array with link data
$ctgData = array (
  0 => array (
    'lname' => 'Root',
    'lurl' => '#',
  ),
  1 => array (
    'lname' => 'CoursesWeb.net',
    'lurl' => 'https://coursesweb.net/',
  ),
  2 => array (
    'lname' => 'JavaScript Courses',
    'lurl' => 'https://coursesweb.net/javascript/',
  ),
  3 => array (
    'lname' => 'Tutorials',
    'lurl' => 'https://coursesweb.net/javascript/tutorials_t',
  ),
  4 => array (
    'lname' => 'setTimeout - setInterva',
    'lurl' => 'https://coursesweb.net/javascript/settimeout-setinterval_t',
  ),
  5 => array (
    'lname' => 'Dynamic variables',
    'lurl' => 'https://coursesweb.net/javascript/dynamic-variables-javascript_t',
  ),
  6 => array (
    'lname' => 'Validate radio - checkbox',
    'lurl' => 'https://coursesweb.net/javascript/validate-radio-checkbox-buttons_t',
  ),
  7 => array (
    'lname' => 'jQuery Course',
    'lurl' => 'https://coursesweb.net/jquery/jquery-course',
  ),
  8 => array (
    'lname' => 'jQuery Basics',
    'lurl' => 'https://coursesweb.net/jquery/jquery-basics',
  ),
  9 => array (
    'lname' => 'Selecting HTML elements',
    'lurl' => 'https://coursesweb.net/jquery/jquery-basics#selelm',
  ),
  10 => array (
    'lname' => 'jQuery ajax()',
    'lurl' => 'https://coursesweb.net/jquery/jquery-ajax',
  ),
  11 => array (
    'lname' => 'slideDown - SlideUp',
    'lurl' => 'https://coursesweb.net/jquery/slidedown-slideup',
  ),
  12 => array (
    'lname' => 'JS Scripts',
    'lurl' => 'https://coursesweb.net/javascript/javascript-scripts_s2',
  ),
  13 => array (
    'lname' => 'JS Trivia Game',
    'lurl' => 'https://coursesweb.net/javascript/trivia-game-script_s2',
  ),
  14 => array (
    'lname' => 'Scroll to Top Page Button',
    'lurl' => 'https://coursesweb.net/javascript/trivia-game-script_s2',
  ),
  15 => array (
    'lname' => 'PHP-MySQL Course',
    'lurl' => 'https://coursesweb.net/php-mysql/',
  ),
  16 => array (
    'lname' => 'Lessons',
    'lurl' => 'https://coursesweb.net/php-mysql/lessons',
  ),
  17 => array (
    'lname' => 'PHP Strings',
    'lurl' => 'https://coursesweb.net/php-mysql/strings',
  ),
  18 => array (
    'lname' => 'Constants',
    'lurl' => 'https://coursesweb.net/php-mysql/constants',
  ),
  19 => array (
    'lname' => 'Arrays',
    'lurl' => 'https://coursesweb.net/php-mysql/arrays',
  ),
  20 => array (
    'lname' => 'PHP PDO',
    'lurl' => 'https://coursesweb.net/php-mysql/pdo-introduction-connection-database',
  ),
  21 => array (
    'lname' => 'Tutorials',
    'lurl' => 'https://coursesweb.net/php-mysql/tutorials_t',
  ),
  22 => array (
    'lname' => 'Uploading multiple files',
    'lurl' => 'https://coursesweb.net/php-mysql/uploading-multiple-files_t',
  ),
  23 => array (
    'lname' => 'Common PHP Errors',
    'lurl' => 'https://coursesweb.net/php-mysql/common-php-errors-solution_t',
  ),
  24 => array (
    'lname' => 'Unespected /',
    'lurl' => 'https://coursesweb.net/php-mysql/common-php-errors-solution_t#perr1_0',
  ),
  25 => array (
    'lname' => 'headers already sent',
    'lurl' => 'https://coursesweb.net/php-mysql/common-php-errors-solution_t#perr12_11',
  ),
  26 => array (
    'lname' => 'Undefined variable',
    'lurl' => 'https://coursesweb.net/php-mysql/common-php-errors-solution_t#perr13_12',
  ),
  27 => array (
    'lname' => 'Code Snippets',
    'lurl' => 'https://coursesweb.net/php-mysql/common-php-errors-solution_t#perr13_12',
  ),
  28 => array (
    'lname' => 'Ajax Lessons',
    'lurl' => 'https://coursesweb.net/ajax/',
  ),
  29 => array (
    'lname' => 'Tutorials',
    'lurl' => 'https://coursesweb.net/ajax/tutorials_t',
  ),
  30 => array (
    'lname' => 'Download Resources',
    'lurl' => 'https://coursesweb.net/ajax/download_l2',
  ),
  31 => array (
    'lname' => 'HTML Course',
    'lurl' => 'https://coursesweb.net/html/',
  ),
  32 => array (
    'lname' => 'CSS Course',
    'lurl' => 'https://coursesweb.net/css/',
  ),
  33 => array (
    'lname' => 'MarPlo.net',
    'lurl' => 'https://marplo.net/',
  ),
  34 => array (
    'lname' => 'JavaScript',
    'lurl' => 'https://marplo.net/javascript/',
  ),
  35 => array (
    'lname' => 'JS Lessons',
    'lurl' => 'https://marplo.net/javascript/lectii-js',
  ),
  36 => array (
    'lname' => 'Syntax',
    'lurl' => 'https://marplo.net/javascript/sintaxajs.html',
  ),
  37 => array (
    'lname' => 'Functions',
    'lurl' => 'https://marplo.net/javascript/functii1.html',
  ),
  38 => array (
    'lname' => 'jQuery',
    'lurl' => 'https://marplo.net/javascript/curs-jquery-tutoriale-js',
  ),
  39 => array (
    'lname' => 'jQuery CSS',
    'lurl' => 'https://marplo.net/javascript/jquery-css-js',
  ),
  40 => array (
    'lname' => 'CSS Properties',
    'lurl' => 'https://marplo.net/javascript/jquery-css-js#setcss',
  ),
  41 => array (
    'lname' => 'Add / Delete Classes',
    'lurl' => 'https://marplo.net/javascript/jquery-css-js#arclass',
  ),
  42 => array (
    'lname' => 'Check Classes',
    'lurl' => 'https://marplo.net/javascript/jquery-css-js#ceclass',
  ),
  43 => array (
    'lname' => 'fadeIn - fadeOut',
    'lurl' => 'https://marplo.net/javascript/jquery-fadein-fadeout-js',
  ),
  44 => array (
    'lname' => 'fadeIn - fadeOut',
    'lurl' => 'https://marplo.net/javascript/jquery-fadein-fadeout-js',
  ),
  45 => array (
    'lname' => 'fadeTo',
    'lurl' => 'https://marplo.net/javascript/jquery-fadein-fadeout-js#fadeto',
  ),
  46 => array (
    'lname' => 'JS Tutorials',
    'lurl' => 'https://marplo.net/javascript/tutoriale-js',
  )
);

echo '<div id="m_menu">'. multilevelMenu(0, $ctgLists, $ctgData) .'</div>';     // outputs html with menu lists
?>
The code above will generate these HTML lists:

- Once the HTML code with the lists is created, you can apply CSS properties to design the dropdown menu.
Here's two CSS codes that can be used for Horizontal and Vertical Dropdown Menu. The menu-lists are included into a DIV with id="m_menu" (click on the image to see how the menu looks):

CSS code for Horizontal-Vertical Menu

Horizontal-Vertical menu

CSS code for Vertical-Horizontal Menu

Vertical-Horizontal menu

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
Which tag is used to add definition lists into a <dl> element?
<dt> <dd> <li>
<dl>
 <dt>HTML</dt>
  <dd> - Hyper Text Markup Language</dd>
  <dd> - Language for web pages</dd>
</dl>
Which CSS property can hide an element on page, letting an empty space in its place?
display position visibility
#id {
  visibility: hidden;
}
Click on the event which is triggered when the mouse clicks on an object.
onclick onmouseover onfocus
document.getElementById("id").onclick = function(){
  alert("http://CoursesWeb.net/");
}
Indicate the PHP variable that contains the contents of both $_GET, $_POST, and $_COOKIE arrays.
$_SESSION $_GET $_REQUEST
if(isset($_REQUEST["id"])) {
  echo $_REQUEST["id"];
}
Recursive function to create Multi-Level Menu in PHP

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