in_category() WP 1.0

Check if the current post is within any of the given categories.

The given categories are checked against the post's categories' term_ids, names and slugs. Categories given as integers will only be checked against the post's categories' term_ids.

Prior to v2.5 of WordPress, category names were not supported. Prior to v2.7, category slugs were not supported. Prior to v2.7, only one category could be compared: in_category( $single_category ). Prior to v2.7, this function could only be used in the WordPress Loop. As of 2.7, the function can be used anywhere if it is provided a post ID or post object.

No Hooks.

Return

true/false. True if the current post is in any of the given categories.

Usage

in_category( $category, $post );

$category(int/string/array) (required)

Category ID, name or slug, or array of said.

$post(int/object)

Post to check instead of the current post. (since 2.7.0)
Default: null

Code of in_category: wp-includes/category-template.php ^{VER 4.9.8}

<?php
function in_category( $category, $post = null ) {
	if ( empty( $category ) )
		return false;

	return has_category( $category, $post );
}

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