Index: Expr.php
===================================================================
--- Expr.php	(revision 14818)
+++ Expr.php	(working copy)
@@ -86,6 +86,7 @@
 		'div' => EXPR_DIVIDE
 	);
 
+	var $_bcmath_enabled;
 
 	/**
 	 * Add expression messages to the message cache
@@ -353,7 +354,19 @@
 				$right = array_pop( $stack );
 				$left = array_pop( $stack );
 				if ( $right == 0 ) return 'division_by_zero';
-				$stack[] = $left % $right;
+				
+				// check if BCMath is enabled and cache the result
+				if ($this->_bcmath_enabled === null) {
+					$this->_bcmath_enabled = extension_loaded('bcmath'); 
+				}
+				
+				// call BCMath if it's enabled to fix Bug 6356
+				if ($this->_bcmath_enabled) {
+					$stack[] = bcmod($left, $right);
+				} else
+					$stack[] = $left % $right;
+				}
+				
 				break;
 			case EXPR_PLUS:
 				if ( count( $stack ) < 2 ) return 'missing_operand';