How to make a function work on a specific page?

Question

PHP

I have a function that displays the subcategories on the category page, but the problem is that it works on the main page as well and displays the categories mixed with the subcategories. Here is the code:

$i=1;
$cat = get_query_var('cat');
$categories = get_categories('parent='.$cat.''); 
foreach ($categories as $category) { $i++; }
if ($i > 1) {
  echo "<ul>";
  foreach ($categories as $category) { ?>
    <li><a href="<?php echo get_category_link($category->term_id); ?>" ><?php echo $category->name; ?></a></li>
  <?php } 
  echo "</ul>";
} else {
  $pcat = get_category(get_query_var('cat'),false);
  $pcatid = $pcat->category_parent;
  $categories = get_categories('parent='.$pcatid.''); 
  echo "<ul>";
  foreach ($categories as $category) { ?>
    <li<?php if ($category->term_id == $cat) { ?> class="active"<?php } ?>><a href="<?php echo get_category_link($category->term_id); ?>" ><?php echo $category->name; ?></a></li>
  <?php } 
  echo "</ul>";
}

I put it in the index.php file, but I need it to work only in the category page, and not on the main one.

Answer 1

I created a category.php file and inserted all the code from index.php into it along with my function, and removed it from index.php and everything worked.

Answer 2

put it on the category page?! Of course, you can play around with the url definition or load the script in php if !==home