Add hash_equals() function

includes/wc-core-functions.php

@@ -737,3 +737,33 @@ function wc_get_customer_default_location() {
 
 	return $location;
 }
+
+// This function can be removed when WP 3.9.2 or greater is required
+if ( ! function_exists( 'hash_equals' ) ) :
+	/**
+	 * Compare two strings in constant time.
+	 *
+	 * This function was added in PHP 5.6.
+	 * It can leak the length of a string.
+	 *
+	 * @since 3.9.2
+	 *
+	 * @param string $a Expected string.
+	 * @param string $b Actual string.
+	 * @return bool Whether strings are equal.
+	 */
+	function hash_equals( $a, $b ) {
+		$a_length = strlen( $a );
+		if ( $a_length !== strlen( $b ) ) {
+			return false;
+		}
+		$result = 0;
+
+		// Do not attempt to "optimize" this.
+		for ( $i = 0; $i < $a_length; $i++ ) {
+			$result |= ord( $a[ $i ] ) ^ ord( $b[ $i ] );
+		}
+
+		return $result === 0;
+	}
+endif;