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WP_Query::is_tag() WP 3.1.0

Is the query for an existing tag archive page?

If the $tag parameter is specified, this function will additionally check if the query is for one of the tags specified.

This is a method of the class: WP_Query

No Hooks.

Return

true/false.

Usage

$WP_Query = new WP_Query();
$var = $WP_Query->is_tag( $tag );
$tag(mixed)
Tag ID, name, slug, or array of Tag IDs, names, and slugs.
Default: ''

Changelog

Since 3.1.0 Introduced.

Code of WP Query::is tag: wp-includes/class-wp-query.php WP 5.2.3

<?php
public function is_tag( $tag = '' ) {
	if ( ! $this->is_tag ) {
		return false;
	}

	if ( empty( $tag ) ) {
		return true;
	}

	$tag_obj = $this->get_queried_object();

	$tag = array_map( 'strval', (array) $tag );

	if ( in_array( (string) $tag_obj->term_id, $tag ) ) {
		return true;
	} elseif ( in_array( $tag_obj->name, $tag ) ) {
		return true;
	} elseif ( in_array( $tag_obj->slug, $tag ) ) {
		return true;
	}

	return false;
}