Get all terms from a taxonomy and use a select options

helper-functions.php

function get_tax_options( $taxonomy, $field ) {
	$terms = get_terms( $taxonomy );
	$current = ( isset($_POST[$field]) ) ? $_POST[$field] : ''; 
	$options = array();
	
	$options[] = '<option value="">-- ' . ucwords( $taxonomy ) .' --</option>';
	if ( ! empty( $terms ) && ! is_wp_error( $terms ) ){
	    foreach ( $terms as $term ) {
	       $options[] = sprintf( '<option value="%s" %s>%s</option>', $term->slug ,selected( $current, $term->slug ), $term->name );  
	    }
	}

	 return implode( ' ', $options );;
}