diff --git a/README.md b/README.md
index b86f9c6..81d9475 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # MathExecutor [![Stories in Ready](https://badge.waffle.io/NeonXP/MathExecutor.png?label=ready&title=Ready)](https://waffle.io/NeonXP/MathExecutor) [![Build Status](https://travis-ci.org/NeonXP/MathExecutor.png?branch=master)](https://travis-ci.org/NeonXP/MathExecutor)
 
-A simple math expressions calculator
+# A simple and extensible math expressions calculator
 
 ## Features:
 * Built in support for +, -, *, / and power (^) operators plus ()
@@ -11,7 +11,7 @@ A simple math expressions calculator
 * Exceptions on divide by zero, or treat as zero
 * Unary Minus (e.g. -3)
 * Pi ($pi) and Euler's number ($e) support to 11 decimal places
-* Easily extendable
+* Easily extensible
 
 ## Install via Composer:
 ```
@@ -41,10 +41,9 @@ Default functions:
 
 Add custom function to executor:
 ```php
-$executor->addFunction('abs', function($arg) {
-    return abs($arg);
-}, 1);
+$executor->addFunction('abs', function($arg) {return abs($arg);});
 ```
+Function default parameters are not supported at this time.
 
 ## Operators:
 Default operators: `+ - * / ^`
diff --git a/src/NXP/Classes/Calculator.php b/src/NXP/Classes/Calculator.php
index 1ceac84..62dbc17 100644
--- a/src/NXP/Classes/Calculator.php
+++ b/src/NXP/Classes/Calculator.php
@@ -38,22 +38,18 @@ public function calculate($tokens, $variables)
         foreach ($tokens as $token) {
             if ($token instanceof TokenNumber) {
                 array_push($stack, $token);
-            }
-            if ($token instanceof TokenStringDoubleQuoted) {
+            } else if ($token instanceof TokenStringDoubleQuoted) {
                 array_push($stack, $token);
-            }
-            if ($token instanceof TokenStringSingleQuoted) {
+            } else if ($token instanceof TokenStringSingleQuoted) {
                 array_push($stack, $token);
-            }
-            if ($token instanceof TokenVariable) {
+            } else if ($token instanceof TokenVariable) {
                 $variable = $token->getValue();
                 if (!array_key_exists($variable, $variables)) {
                     throw new UnknownVariableException($variable);
                 }
                 $value = $variables[$variable];
                 array_push($stack, new TokenNumber($value));
-            }
-            if ($token instanceof InterfaceOperator || $token instanceof TokenFunction) {
+            } else if ($token instanceof InterfaceOperator || $token instanceof TokenFunction) {
                 array_push($stack, $token->execute($stack));
             }
         }
diff --git a/src/NXP/Classes/Token/TokenFunction.php b/src/NXP/Classes/Token/TokenFunction.php
index b2866c3..04eae30 100644
--- a/src/NXP/Classes/Token/TokenFunction.php
+++ b/src/NXP/Classes/Token/TokenFunction.php
@@ -32,9 +32,9 @@ public function execute(&$stack)
         $args = [];
         list($places, $function) = $this->value;
         for ($i = 0; $i < $places; $i++) {
-            array_push($args, array_pop($stack)->getValue());
+            $args[] = array_pop($stack)->getValue();
         }
-        $result = call_user_func_array($function, $args);
+        $result = call_user_func_array($function, array_reverse($args));
 
         return new TokenNumber($result);
     }
diff --git a/src/NXP/MathExecutor.php b/src/NXP/MathExecutor.php
index 29c6a64..6325d35 100644
--- a/src/NXP/MathExecutor.php
+++ b/src/NXP/MathExecutor.php
@@ -174,7 +174,7 @@ public function getOperators()
      * @return MathExecutor
      * @throws \ReflectionException
      */
-    public function addFunction($name, $function = null, $places = 1)
+    public function addFunction($name, $function = null, $places = null)
     {
         $this->tokenFactory->addFunction($name, $function, $places);
 
diff --git a/tests/MathTest.php b/tests/MathTest.php
index 55e0799..8eb7827 100644
--- a/tests/MathTest.php
+++ b/tests/MathTest.php
@@ -32,7 +32,7 @@ public function testCalculating($expression)
 
         /** @var float $phpResult */
         eval('$phpResult = ' . $expression . ';');
-        $this->assertEquals($calculator->execute($expression), $phpResult, "Expression was: ${expression}");
+        $this->assertEquals($phpResult, $calculator->execute($expression), "Expression was: ${expression}");
     }
 
     /**
@@ -117,7 +117,7 @@ public function testIncorrectExpressionException()
     public function testZeroDivision()
     {
         $calculator = new MathExecutor();
-        $this->assertEquals($calculator->execute('10 / 0'), 0);
+        $this->assertEquals(0, $calculator->execute('10 / 0'));
     }
 
     public function testZeroDivisionException()
@@ -131,19 +131,25 @@ public function testZeroDivisionException()
     public function testExponentiation()
     {
         $calculator = new MathExecutor();
-        $this->assertEquals($calculator->execute('10 ^ 2'), 100);
+        $this->assertEquals(100, $calculator->execute('10 ^ 2'));
     }
 
+    public function testFunctionParameterOrder()
+		{
+				$calculator = new MathExecutor();
+
+				$calculator->addFunction('concat', function ($arg1, $arg2) {return $arg1.$arg2;});
+				$this->assertEquals('testing', $calculator->execute('concat("test","ing")'));
+				$this->assertEquals('testing', $calculator->execute("concat('test','ing')"));
+		}
+
     public function testFunction()
     {
         $calculator = new MathExecutor();
-
-        $calculator->addFunction('round', function ($arg) {
-            return round($arg);
-        }, 1);
+        $calculator->addFunction('round', function ($arg) {return round($arg);});
         /** @var float $phpResult */
         eval('$phpResult = round(100/30);');
-        $this->assertEquals($calculator->execute('round(100/30)'), $phpResult);
+        $this->assertEquals($phpResult, $calculator->execute('round(100/30)'));
     }
 
     public function testQuotes()
@@ -151,9 +157,9 @@ public function testQuotes()
         $calculator = new MathExecutor();
         $testString = "some, long. arg; with: different-separators!";
         $calculator->addFunction('test', function ($arg) use ($testString) {
-            $this->assertEquals($arg, $testString);
-            return 0;
-        }, 1);
+            $this->assertEquals($testString, $arg);
+            return 0;}
+        );
         $calculator->execute('test("' . $testString . '")'); // single quotes
         $calculator->execute("test('" . $testString . "')"); // double quotes
     }