diff --git a/src/NXP/Classes/Calculator.php b/src/NXP/Classes/Calculator.php
index bcb4b4e..49142df 100644
--- a/src/NXP/Classes/Calculator.php
+++ b/src/NXP/Classes/Calculator.php
@@ -49,7 +49,7 @@ public function __construct(array $functions, array $operators)
      * @throws IncorrectExpressionException
      * @throws UnknownVariableException
      */
-    public function calculate(array $tokens, array $variables)
+    public function calculate(array $tokens, array $variables, callable $onVarNotFound = null)
     {
         /** @var Token[] $stack */
         $stack = [];
@@ -58,10 +58,18 @@ public function calculate(array $tokens, array $variables)
                 $stack[] = $token;
             } elseif ($token->type === Token::Variable) {
                 $variable = $token->value;
-                if (!array_key_exists($variable, $variables)) {
+
+                $value = null;
+                if (array_key_exists($variable, $variables)) {
+                    $value = $variables[$variable];
+                } elseif ($onVarNotFound) {
+                    $value = call_user_func($onVarNotFound, $variable);
+                }
+
+                if (!isset($value)) {
                     throw new UnknownVariableException($variable);
                 }
-                $value = $variables[$variable];
+
                 $stack[] = new Token(Token::Literal, $value);
             } elseif ($token->type === Token::Function) {
                 if (!array_key_exists($token->value, $this->functions)) {
diff --git a/src/NXP/Exception/MathExecutorException.php b/src/NXP/Exception/MathExecutorException.php
index 0e3ea84..4405f85 100644
--- a/src/NXP/Exception/MathExecutorException.php
+++ b/src/NXP/Exception/MathExecutorException.php
@@ -14,6 +14,6 @@
 /**
  * @author Vitaliy Zhuk <zhuk2205@gmail.com>
  */
-abstract class MathExecutorException extends \Exception
+class MathExecutorException extends \Exception
 {
 }
diff --git a/src/NXP/MathExecutor.php b/src/NXP/MathExecutor.php
index 8f876dc..d23ba80 100644
--- a/src/NXP/MathExecutor.php
+++ b/src/NXP/MathExecutor.php
@@ -15,9 +15,9 @@
 use NXP\Classes\CustomFunction;
 use NXP\Classes\Operator;
 use NXP\Classes\Tokenizer;
-use NXP\Exception\MathExecutorException;
 use NXP\Exception\DivisionByZeroException;
 use NXP\Exception\UnknownVariableException;
+use NXP\Exception\MathExecutorException;
 use ReflectionException;
 
 /**
@@ -33,6 +33,11 @@ class MathExecutor
      */
     private $variables = [];
 
+    /**
+     * @var callable
+     */
+    private $onVarNotFound = null;
+
     /**
      * @var Operator[]
      */
@@ -134,14 +139,22 @@ function ($a, $b) {
             ],
             '==' => [
                 function ($a, $b) {
-                    return $a == $b;
+                    if (is_string($a) || is_string($b)) {
+                        return strcmp($a, $b) == 0;
+                    } else {
+                        return $a == $b;
+                    }
                 },
                 140,
                 false
             ],
             '!=' => [
                 function ($a, $b) {
-                    return $a != $b;
+                    if (is_string($a) || is_string($b)) {
+                        return strcmp($a, $b) != 0;
+                    } else {
+                        return $a != $b;
+                    }
                 },
                 140,
                 false
@@ -353,7 +366,7 @@ public function execute(string $expression)
             $tokens = $this->cache[$cachekey];
         }
         $calculator = new Calculator($this->functions, $this->operators);
-        return $calculator->calculate($tokens, $this->variables);
+        return $calculator->calculate($tokens, $this->variables, $this->onVarNotFound);
     }
 
     /**
@@ -415,13 +428,13 @@ public function getVar(string $variable)
      * @param  string $variable
      * @param  integer|float $value
      * @return MathExecutor
-     * @throws MathExecutorException
      */
     public function setVar(string $variable, $value) : self
     {
-        if (!is_numeric($value)) {
-            throw new MathExecutorException("Variable ({$variable}) value must be a number ({$value}) type ({gettype($value)})");
+        if (!is_scalar($value)) {
+            throw new MathExecutorException("Variable ({$variable}) value must be a scalar type ({gettype($value)})");
         }
+
         $this->variables[$variable] = $value;
         return $this;
     }
@@ -445,6 +458,20 @@ public function setVars(array $variables, bool $clear = true) : self
         return $this;
     }
 
+    /**
+     * Define a method that will be invoked when a variable is not found.
+     * The first parameter will be the variable name, and the returned value will be used as the variable value.
+     *
+     * @param callable $handler
+     *
+     * @return MathExecutor
+     */
+    public function setVarNotFoundHandler(callable $handler): self
+    {
+        $this->onVarNotFound = $handler;
+        return $this;
+    }
+
     /**
      * Remove variable from executor
      *
@@ -458,12 +485,13 @@ public function removeVar(string $variable) : self
     }
 
     /**
-     * Remove all variables
+     * Remove all variables and the variable not found handler
      * @return MathExecutor
      */
     public function removeVars() : self
     {
         $this->variables = [];
+        $this->onVarNotFound = null;
         return $this;
     }
 
@@ -474,7 +502,7 @@ public function removeVars() : self
      */
     public function getOperators()
     {
-        return $this->tokenFactory->getOperators();
+        return $this->operators;
     }
 
     /**
@@ -485,7 +513,7 @@ public function getOperators()
      */
     public function getFunctions() : array
     {
-        return $this->tokenFactory->getFunctions();
+        return $this->functions;
     }
 
     /**
diff --git a/tests/MathTest.php b/tests/MathTest.php
index ee9c6bd..d76a57c 100644
--- a/tests/MathTest.php
+++ b/tests/MathTest.php
@@ -409,6 +409,37 @@ public function testBeginWithBracketAndMinus()
         $this->assertEquals(1, $calculator->execute('(-4 + 5)'));
     }
 
+    public function testStringComparison()
+    {
+        $calculator = new MathExecutor();
+        $this->assertEquals(true, $calculator->execute('"a" == \'a\''));
+        $this->assertEquals(true, $calculator->execute('"hello world" == "hello world"'));
+        $this->assertEquals(false, $calculator->execute('"hello world" == "hola mundo"'));
+        $this->assertEquals(true, $calculator->execute('"hello world" != "hola mundo"'));
+    }
+
+    public function testVarStringComparison()
+    {
+        $calculator = new MathExecutor();
+        $calculator->setVar('var', 0);
+        $this->assertEquals($calculator->execute('0 == "a"'), $calculator->execute('var == "a"'));
+    }
+
+    public function testOnVarNotFound()
+    {
+        $calculator = new MathExecutor();
+        $calculator->setVarNotFoundHandler(
+            function ($varName) {
+                if ($varName == 'undefined') {
+                    return 3;
+                } else {
+                    return null;
+                }
+            }
+        );
+        $this->assertEquals(15, $calculator->execute('5 * undefined'));
+    }
+
     public function testMinusZero()
     {
         $calculator = new MathExecutor();