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<?php
add_filter( 'woocommerce_product_tabs', 'woo_reorder_tabs', 98 );
function woo_reorder_tabs( $tabs ) {

	$tabs['description']['priority'] = 5;			// Description second
	$tabs['additional_information']['priority'] = 10;			// Description second
	$tabs['test_tab']['priority'] = 15;	// Additional information third
	$tabs['reviews']['priority'] = 20;			// Reviews first

	return $tabs;
}