non-hierarchical-posts-dropdown.php

<?php 
	/**
	 * Display a dropdown of non-hierarchical post types. 
	 * For hierarchical CPTs you can use `wp_dropdown_pages`. 
	 *
	 * @param string $args Arguments.
	 *
	 * @return void
	 */
	public function dropdown_non_hierarchical_posts( $args ) {
		$defaults = array(
			'show_option_none' => '',
			'name'             => '',
			'selected'         => '',
			'post_type'        => '',
		);

		$args = wp_parse_args( $args, $defaults );

		if ( empty( $args['post_type'] ) ) {
			return;
		}

		$post_items = get_posts(
			array(
				'post_type'        => $args['post_type'],
				'post_status'      => 'publish',
				'suppress_filters' => false,
				'posts_per_page'   => -1,
			)
		);

		if ( empty( $post_items ) ) {
			return;
		}

		?>
		<select name="<?php echo esc_attr( $args['name'] ); ?>" id="<?php echo esc_attr( $args['name'] ); ?>">
			<option value="0"><?php echo esc_html( $args['show_option_none'] ); ?></option>
			<?php
			foreach ( $post_items as $_post ) {
				$selected = intval( $args['selected'] ) === $_post->ID ? ' selected=' : '';
				echo sprintf( '<option value="%d" %s>%s</optio>', esc_attr( $_post->ID ), esc_attr( $selected ), esc_html( $_post->post_title ) );
			}
			?>
		</select>
		<?php
	}