the_category_ID()

the_category_ID( bool $display = true ): int

function the_category_ID($display = true) {
	_deprecated_function( __FUNCTION__, '0.71', 'get_the_category()' );

	// Grab the first cat in the list.
	$categories = get_the_category();
	$cat = $categories[0]->term_id;

	if ( $display )
		echo $cat;

	return $cat;
}