how to get the active post’s category

In a category template:

$cat = get_query_var('cat');
echo $cat;

In a single type template and in The Loop see example in the article get_the_category.

okay thanks

I now get the cat ID using this piece of code:

$category = get_the_category();
echo $category[0]->cat_ID;

but this

if ($category[0]->cat_ID = 113) {
...
} elseif ($category[0]->cat_ID = 122) {
...
}

wont work..

kay thanks

this works:

$category = get_the_category();
$myCat = $category[0]->cat_ID;
if ($myCat == 113) {
...
} elseif ($myCat == 122) {
...
}

Its because you arent using an operator, you are checking to see if the category was assigned. This is what you should be doing:

$category = get_the_category();
if ( $category[0]->cat_ID == 3 )
{
	soSomething();
}