wp_is_internal_link() │ WP 6.2.0

Determines whether or not the specified URL is of a host included in the internal hosts list.

No Hooks.

Returns

true|false. Returns true for internal URLs and false for all other URLs.

Usage

wp_is_internal_link( $link );
$link(string) (required)
The URL to test.

Notes

Changelog

Since 6.2.0 Introduced.

wp_is_internal_link() code WP 7.0

wp-includes/link-template.php 
function wp_is_internal_link( $link ) {
	$link = strtolower( $link );
	if ( in_array( wp_parse_url( $link, PHP_URL_SCHEME ), wp_allowed_protocols(), true ) ) {
		return in_array( wp_parse_url( $link, PHP_URL_HOST ), wp_internal_hosts(), true );
	}
	return false;
}