Improve example: is_callable() and constructors (#4274)

Author: kamil-tekiela

reference/var/functions/is-callable.xml

@@ -133,8 +133,11 @@ string(21) "SomeClass::someMethod"
    <example>
     <title><function>is_callable</function> and constructors</title>
     <simpara>
-     <function>is_callable</function> reports constructors as
-     not being callable.
+     Despite the fact that constructors are the methods that are called when
+     an object is created, they are not static methods and
+     <function>is_callable</function> will return &false; for them. It's not
+     possible to use <function>is_callable</function> to check if a class can
+     be instantiated from the current scope.
     </simpara>
     <programlisting role="php">
 <![CDATA[
@@ -152,6 +155,9 @@ var_dump(
     is_callable(['Foo', 'foo'])
 );
 
+$foo = new Foo();
+var_dump(is_callable([$foo, '__construct']));
+
 ?>
 ]]>
     </programlisting>
@@ -160,6 +166,7 @@ var_dump(
 <![CDATA[
 bool(false)
 bool(false)
+bool(true)
 ]]>
     </screen>
    </example>