Description:
------------
When ReflectionParameter::isOptional() returns false, because other parameters 
after it are required, isDefaultValueAvailable() always return false, even is 
there actually is a default value.

I've read this older bug report: https://bugs.php.net/bug.php?id=41382
It looks like at that time, this behaviour had been considerer consistent.
I do think this should be revisited however, as in the example function below, 
there is absolutely no way with Reflection to get the default value for the 
given parameter.

Reflection is a powerful tool for autowiring in Dependency Injection containers, 
where it is interesting to check whether default parameters are available, 
regardless of their order.


Test script:
---------------
function test(PDO $a = null, $b = 0, array $c) {}
$r = new ReflectionFunction('test');

foreach ($r->getParameters() as $p) {
    echo $p->getName();
    echo "   isDefaultValueAvailable: " . var_export($p->isDefaultValueAvailable(), true) . "\n";
    echo "    isOptional: " . var_export($p->isOptional(), true) . "\n";
    echo "    allowsNull: " . var_export($p->allowsNull(), true) . "\n";
    echo "\n";
}

Expected result:
----------------
a   isDefaultValueAvailable: true
    isOptional: false
    allowsNull: true

b   isDefaultValueAvailable: true
    isOptional: false
    allowsNull: true

c   isDefaultValueAvailable: false
    isOptional: false
    allowsNull: false

Actual result:
--------------
a   isDefaultValueAvailable: false
    isOptional: false
    allowsNull: true

b   isDefaultValueAvailable: false
    isOptional: false
    allowsNull: true

c   isDefaultValueAvailable: false
    isOptional: false
    allowsNull: false