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/**
 * woocommerce_package_rates is a 2.1+ hook
 */
add_filter( 'woocommerce_package_rates', 'hide_shipping_when_free_is_available', 10, 2 );
 
/**
 * Hide shipping rates when free shipping is available
 *
 * @param array $rates Array of rates found for the package
 * @param array $package The package array/object being shipped
 * @return array of modified rates
 */
function hide_shipping_when_free_is_available( $rates, $package ) {
 	
 	// Only modify rates if free_shipping is present
  	if ( isset( $rates['free_shipping'] ) ) {
  	
  		// To unset a single rate/method, do the following. This example unsets flat_rate shipping
  		unset( $rates['flat_rate'] );
  		
  		// To unset all methods except for free_shipping, do the following
  		$free_shipping          = $rates['free_shipping'];
  		$rates                  = array();
  		$rates['free_shipping'] = $free_shipping;
	}
	
	return $rates;
}