the_category_ID( bool $echo = true )

Source

File: wp-includes/deprecated.php

function the_category_ID($echo = 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 ( $echo )
		echo $cat;

	return $cat;
}