get_term_by() │ WP 2.3.0

Get all Term data from database by Term field and data.

Warning: $value is not escaped for 'name' $field. You must do it yourself, if required.

The default $field is 'id', therefore it is possible to also use null for field, but not recommended that you do so.

If $value does not exist, the return value will be false. If $taxonomy exists and $field and $value combinations exist, the Term will be returned.

This function will always return the first term that matches the $field- $value-$taxonomy combination specified in the parameters. If your query is likely to match more than one term (as is likely to be the case when $field is 'name', for example), consider using get_terms() instead; that way, you will get all matching terms, and can provide your own logic for deciding which one was intended.

Uses: get_terms(), get_term()
Used By: get_cat_ID(), get_category_by_slug(), wp_get_nav_menu_object()
1 time — 0.001272 sec (very slow) | 50000 times — 4.95 sec (fast) | PHP 7.1.11, WP 4.9.6

No Hooks.

Return

WP_Term|Array|false. WP_Term instance (or array) on success, depending on the $output value. False if $taxonomy does not exist or $term was not found.

Usage

get_term_by( $field, $value, $taxonomy, $output, $filter );
$field(string) (required)
Either 'slug', 'name', 'term_id' (or 'id', 'ID'), or 'term_taxonomy_id'.
$value(string|int) (required)
Search for this term value.
$taxonomy(string)
Taxonomy name. Optional, if $field is 'term_taxonomy_id'.
Default: ''
$output(string)
The required return type. One of OBJECT, ARRAY_A, or ARRAY_N, which correspond to a WP_Term object, an associative array, or a numeric array, respectively.
Default: OBJECT
$filter(string)
How to sanitize term fields.
Default: 'raw'

Notes

Changelog

Since 2.3.0 Introduced.
Since 4.4.0 $taxonomy is optional if $field is 'term_taxonomy_id'. Converted to return a WP_Term object if $output is OBJECT.
Since 5.5.0 Added 'ID' as an alias of 'id' for the $field parameter.

Code of get_term_by() WP 5.9.3

wp-includes/taxonomy.php 
function get_term_by( $field, $value, $taxonomy = '', $output = OBJECT, $filter = 'raw' ) {

	// 'term_taxonomy_id' lookups don't require taxonomy checks.
	if ( 'term_taxonomy_id' !== $field && ! taxonomy_exists( $taxonomy ) ) {
		return false;
	}

	// No need to perform a query for empty 'slug' or 'name'.
	if ( 'slug' === $field || 'name' === $field ) {
		$value = (string) $value;

		if ( 0 === strlen( $value ) ) {
			return false;
		}
	}

	if ( 'id' === $field || 'ID' === $field || 'term_id' === $field ) {
		$term = get_term( (int) $value, $taxonomy, $output, $filter );
		if ( is_wp_error( $term ) || null === $term ) {
			$term = false;
		}
		return $term;
	}

	$args = array(
		'get'                    => 'all',
		'number'                 => 1,
		'taxonomy'               => $taxonomy,
		'update_term_meta_cache' => false,
		'orderby'                => 'none',
		'suppress_filter'        => true,
	);

	switch ( $field ) {
		case 'slug':
			$args['slug'] = $value;
			break;
		case 'name':
			$args['name'] = $value;
			break;
		case 'term_taxonomy_id':
			$args['term_taxonomy_id'] = $value;
			unset( $args['taxonomy'] );
			break;
		default:
			return false;
	}

	$terms = get_terms( $args );
	if ( is_wp_error( $terms ) || empty( $terms ) ) {
		return false;
	}

	$term = array_shift( $terms );

	// In the case of 'term_taxonomy_id', override the provided `$taxonomy` with whatever we find in the DB.
	if ( 'term_taxonomy_id' === $field ) {
		$taxonomy = $term->taxonomy;
	}

	return get_term( $term, $taxonomy, $output, $filter );
}

Term (taxonomies terms)

Taxonomies: categories, tags, ...