Old constructors also count as contructor:
<?php
class SomeClass {
    function SomeClass($some_arg) {
    }
}
$refl = new ReflectionClass('SomeClass');
var_dump($refl->isInstantiable()); echo $refl->getConstructor();
?>
Some more behavior:
<?php
class SomeClass {
   function funcA($arg1, $arg2) {
   }
}
$refl = new ReflectionClass('SomeClass');
var_dump($refl->isInstantiable()); var_dump($refl->getConstructor()); class AnotherClass {
   private function __construct() {
   }
   function funcB($arg1, $arg2) {
   }
}
$refl = new ReflectionClass('AnotherClass');
var_dump($refl->isInstantiable()); echo $refl->getConstructor();
?>
Tested on PHP 5.2.4