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// rename the "Have a Coupon?" message on the checkout page
function woocommerce_rename_coupon_message_on_checkout() {
 
	return 'Have a Promo Code?' . ' <a href="#" class="showcoupon">' . __( 'Click here to enter your code', 'woocommerce' ) . '</a>';
}
add_filter( 'woocommerce_checkout_coupon_message', 'woocommerce_rename_coupon_message_on_checkout' );
 
// rename the coupon field on the checkout page
function woocommerce_rename_coupon_field_on_checkout( $translated_text, $text, $text_domain ) {
 
	// bail if not modifying frontend woocommerce text
	if ( is_admin() || 'woocommerce' !== $text_domain ) {
		return $translated_text;
	}
 
	if ( 'Coupon code' === $text ) {
		$translated_text = 'Promo Code';
	
	} elseif ( 'Apply coupon' === $text ) {
		$translated_text = 'Apply Promo Code';
	}
 
	return $translated_text;
}
add_filter( 'gettext', 'woocommerce_rename_coupon_field_on_checkout', 10, 3 );