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<?php
/**
 * Hide the "In stock" message on product page.
 *
 * @param string $html
 * @param string $text
 * @param WC_Product $product
 * @return string
 */
function my_wc_hide_in_stock_message( $html, $text, $product ) {
	$availability = $product->get_availability();

	if ( isset( $availability['class'] ) && 'in-stock' === $availability['class'] ) {
		return '';
	}

	return $html;
}

add_filter( 'woocommerce_stock_html', 'my_wc_hide_in_stock_message', 10, 3 );