create new ul after every 13 li elements - php

Question

First of all, I don't know how to count in php, maybe someone could recommend me a good source to read;
Second, I'm not asking to solve this for me, but I just want a hint or simpler explanation that would make sense;
Here is my function:
<ul>
<?php foreach ($categories as $category) { ?>
<li>
<p><?php echo $category['name']; ?></p>
<?php if ($category['children']) { ?>
<div>
<?php for ($i = 0; $i < count($category['children']);) { ?>
<?php $j = $i + ceil(count($category['children']) / $category['column']); ?>
<?php for (; $i < $j; $i++) { ?>
<?php if (isset($category['children'][$i])) { ?>
<ul>
<li class="none"><h1><?php echo $category['children'][$i]['name']; ?></h1></li>
<?php if ($category['children'][$i]['children_level2']) { ?>
<?php for ($wi = 0; $wi < count($category['children'][$i]['children_level2']); $wi++) { ?>
<li>
<?php echo $category['children'][$i]['children_level2'][$wi]['name']; ?>
</li>
<?php } ?>
<?php } ?>
</ul>
<?php } ?>
<?php } ?>
<?php } ?>
</div>
<?php } ?>
</li>
<?php } ?>
</ul>
I want this part, every 13 li elements to create a new .ul. ./ul. tags
<?php if (isset($category['children'][$i])) { ?>
<ul>
<li class="none"><h1><?php echo $category['children'][$i]['name']; ?></h1></li>
<?php if ($category['children'][$i]['children_level2']) { ?>
<?php for ($wi = 0; $wi < count($category['children'][$i]['children_level2']); $wi++) { ?>
<li>
<?php echo $category['children'][$i]['children_level2'][$wi]['name']; ?>
</li>
<?php } ?>
<?php } ?>
</ul>
<ul><li>maximum of 13 elements</li></ul>
after 13 <li></li> elements create new <ul></ul> tags and put the 14 <li></li> element into the new <ul></ul> tag
I hope I explained what I want to do, for now I'll be waiting for your answers,
p.s. this is more for my learning skills then actual work, so thanks in advice

Answer 1

It's simple. Use condition
if ($wi && $wi % 13 == 0) {
print '</ul><ul>';
}

Answer 2

In order to count 13 elements, you can use the modulo operator %. The idea is that ($iw % 13) is equal to 0 if the content of $iw is a multiple of 13. (see more about the modulo operation)
With an if statement you can insert </ul><ul> when you need in your for loop.

Answer 3

There's two possibilities I can think of:
you could have a separate counter wich resets to 1 and outputs a <ul> every time it reaches 13
or you could check each time if $wi id divisible by 13:
<?php
for ($wi = 0; $wi < count($category['children'][$i]['children_level2']); $wi++) {
if ($wi && $wi % 13 == 0) {
// output <ul>
}
}
?>
If you're unsure what % means, here's the appropriate PHP manual page