Add attributes to WooCommerce product

sample_add_attributes_to_wc_product.php

/**
 * Create variable product.
 */
$post = array(
	'post_title'   => 'Product with attributes only PHP',
	'post_content' => '',
	'post_status'  => 'publish',
	'post_type'    => "product"
);
$new_post_id = wp_insert_post( $post );
wp_set_object_terms ($new_post_id,'variable','product_type');

/**
 * Add product attribute.
 */
$attr_names = array(
	'Colour'	=> array( 'Red' ),
	'Size'		=> array( 'Small', 'Large' )
);
$attr_data = array();
foreach ( $attr_names as $attr_name => $attr_values ) {
	$attr_sanitized_name = 'pa_' . sanitize_title( $attr_name );
	$attr_data += array(
		$attr_sanitized_name => array(
			'name'			=> $attr_name,
			'value'			=> implode( '|', $attr_values ),
			'is_visible' 	=> 1,
			'is_variation' 	=> 1,
			'is_taxonomy' 	=> 0,
			'position' 		=> 0,
		)
	);
}
update_post_meta( $new_post_id, '_product_attributes', $attr_data, TRUE );