diff --git a/Makefile b/Makefile
index 3b100356..8209a7e3 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
-PREFIX		=	/home/work/
-INCLUDE_DIR     = 	${PREFIX}/include/
-LIBRARY_DIR	=	${PREFIX}/lib/
+PREFIX		=	/usr
+INCLUDE_DIR = 	${PREFIX}/include
+LIBRARY_DIR	=	${PREFIX}/lib
 
 all:
 		cd src; $(MAKE)
diff --git a/include/value.h b/include/value.h
index 9eddf40a..d4fae0b3 100644
--- a/include/value.h
+++ b/include/value.h
@@ -569,17 +569,17 @@ class Value
      *  This call operator is only useful when the variable represents a callable
      *  @return Value
      */
-    Value operator()();
-    Value operator()(Value p0);
-    Value operator()(Value p0, Value p1);
-    Value operator()(Value p0, Value p1, Value p2);
-    Value operator()(Value p0, Value p1, Value p2, Value p3);
-    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4);
-    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5);
-    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6);
-    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7);
-    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7, Value p8);
-    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7, Value p8, Value p9);
+    Value operator()() const;
+    Value operator()(Value p0) const;
+    Value operator()(Value p0, Value p1) const;
+    Value operator()(Value p0, Value p1, Value p2) const;
+    Value operator()(Value p0, Value p1, Value p2, Value p3) const;
+    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4) const;
+    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5) const;
+    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6) const;
+    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7) const;
+    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7, Value p8) const;
+    Value operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7, Value p8, Value p9) const;
 
     /**
      *  Call a method
@@ -636,7 +636,7 @@ class Value
      *  @param  argv        The parameters
      *  @return Value
      */
-    Value exec(int argc, struct _zval_struct ***params);
+    Value exec(int argc, struct _zval_struct ***params) const;
 
     /**
      *  Call method with a number of parameters
diff --git a/src/value.cpp b/src/value.cpp
index fd300d5c..c948dcd4 100644
--- a/src/value.cpp
+++ b/src/value.cpp
@@ -687,7 +687,7 @@ Value Value::operator%(double value)                { return Value(numericValue(
  *  @param  p0-p10          Parameters of the function to be called.
  *  @return Value
  */
-Value Value::operator()()
+Value Value::operator()() const
 {
     // call with zero parameters
     return exec(0, NULL);
@@ -698,7 +698,7 @@ Value Value::operator()()
  *  @param  p0          The first parameter
  *  @return Value
  */
-Value Value::operator()(Value p0)
+Value Value::operator()(Value p0) const
 {
     // array of parameters
     zval **params[1] = { &p0._val };
@@ -713,7 +713,7 @@ Value Value::operator()(Value p0)
  *  @param  p1          The second parameter
  *  @return Value
  */
-Value Value::operator()(Value p0, Value p1)
+Value Value::operator()(Value p0, Value p1) const
 {
     // array of parameters
     zval **params[2] = { &p0._val, &p1._val };
@@ -729,7 +729,7 @@ Value Value::operator()(Value p0, Value p1)
  *  @param  p2          The third parameter
  *  @return Value
  */
-Value Value::operator()(Value p0, Value p1, Value p2)
+Value Value::operator()(Value p0, Value p1, Value p2) const
 {
     // array of parameters
     zval **params[3] = { &p0._val, &p1._val, &p2._val };
@@ -746,7 +746,7 @@ Value Value::operator()(Value p0, Value p1, Value p2)
  *  @param  p3          The fourth parameter
  *  @return Value
  */
-Value Value::operator()(Value p0, Value p1, Value p2, Value p3)
+Value Value::operator()(Value p0, Value p1, Value p2, Value p3) const
 {
     // array of parameters
     zval **params[4] = { &p0._val, &p1._val, &p2._val, &p3._val };
@@ -764,7 +764,7 @@ Value Value::operator()(Value p0, Value p1, Value p2, Value p3)
  *  @param  p4          The fifth parameter
  *  @return Value
  */
-Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4)
+Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4) const
 {
     // array of parameters
     zval **params[5] = { &p0._val, &p1._val, &p2._val, &p3._val, &p4._val };
@@ -783,7 +783,7 @@ Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4)
  *  @param  p5          The sixth parameter
  *  @return Value
  */
-Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5)
+Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5) const
 {
     // array of parameters
     zval **params[6] = { &p0._val, &p1._val, &p2._val, &p3._val, &p4._val, &p5._val };
@@ -803,7 +803,7 @@ Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value
  *  @param  p6          The seventh parameter
  *  @return Value
  */
-Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6)
+Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6) const
 {
     // array of parameters
     zval **params[7] = { &p0._val, &p1._val, &p2._val, &p3._val, &p4._val, &p5._val, &p6._val };
@@ -824,7 +824,7 @@ Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value
  *  @param  p7          The eighth parameter
  *  @return Value
  */
-Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7)
+Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7) const
 {
     // array of parameters
     zval **params[8] = { &p0._val, &p1._val, &p2._val, &p3._val, &p4._val, &p5._val, &p6._val, &p7._val };
@@ -846,7 +846,7 @@ Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value
  *  @param  p8          The ninth parameter
  *  @return Value
  */
-Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7, Value p8)
+Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7, Value p8) const
 {
     // array of parameters
     zval **params[9] = { &p0._val, &p1._val, &p2._val, &p3._val, &p4._val, &p5._val, &p6._val, &p7._val, &p8._val };
@@ -869,7 +869,7 @@ Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value
  *  @param  p9          The tenth parameter
  *  @return Value
  */
-Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7, Value p8, Value p9)
+Value Value::operator()(Value p0, Value p1, Value p2, Value p3, Value p4, Value p5, Value p6, Value p7, Value p8, Value p9) const
 {
     // array of parameters
     zval **params[10] = { &p0._val, &p1._val, &p2._val, &p3._val, &p4._val, &p5._val, &p6._val, &p7._val, &p8._val, &p9._val };
@@ -1090,7 +1090,7 @@ Value Value::call(const std::string &name, Value p0, Value p1, Value p2, Value p
  *  @param  argv        The parameters
  *  @return Value
  */
-Value Value::exec(int argc, zval ***params)
+Value Value::exec(int argc, zval ***params) const
 {
     // the return zval
     zval *retval = nullptr;