(PHP 5 >= 5.0.3)
ReflectionParameter::getDefaultValue — Gets default parameter value
Gets the default value of the parameter for a user-defined function or method. If the parameter is not optional a ReflectionException will be thrown.
This function has no parameters.
The parameters default value.
Example #1 Getting
<?php
function foo($test, $bar = 'baz')
{
    echo $test . $bar;
}
$function = new ReflectionFunction('foo');
foreach ($function->getParameters() as $param) {
    echo 'Name: ' . $param->getName() . PHP_EOL;
    if ($param->isOptional()) {
        echo 'Default value: ' . $param->getDefaultValue() . PHP_EOL;
    }
    echo PHP_EOL;
}
?>
The above example will output:
Name: test Name: bar Default value: baz
Note:
Due to implementation details, it is not possible to get the default value of built-in functions or methods of built-in classes. Trying to do this will result a ReflectionException being thrown.