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<?php
/**
 * get_product_id_by_sku 
 * @param  string $sku product sku
 * @return mixed  product id when product found else null
 */
function get_product_id_by_sku( $sku ) {
  global $wpdb;
  	$product_id = $wpdb->get_var( 
	  	$wpdb->prepare( 
	  		"SELECT post_id 
	  		FROM $wpdb->postmeta 
	  		WHERE meta_key='_sku' 
	  		AND meta_value='%s' 
	  		LIMIT 1", 
	  		$sku 
	  	)
  	);
  return ( $product_id )?  $product_id : null;
}