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filter is not described

woocommerce_product_loop_start filter-hook . WC 1.0

Usage

add_filter( 'woocommerce_product_loop_start', 'filter_function_name_1049' );
function filter_function_name_1049( $ob_get_clean ){
	// filter...

	return $ob_get_clean;
}
$ob_get_clean
-

Where the hook is called

woocommerce_product_loop_start()
woocommerce_product_loop_start
woocommerce/includes/wc-template-functions.php 1125
$loop_start = apply_filters( 'woocommerce_product_loop_start', ob_get_clean() );

Where in WP core the hook is used WooCommerce

woocommerce/includes/wc-template-hooks.php 68
add_filter( 'woocommerce_product_loop_start', 'woocommerce_maybe_show_product_subcategories' );