(PHP 5 >= 5.4.6, PHP 7, PHP 8)
ReflectionParameter::getDefaultValueConstantName — Returns the default value's constant name if default value is constant or null
Returns the default value's constant name of the parameter of any user-defined or internal function or method, if default value is constant or null. If the parameter is not optional a ReflectionException will be thrown.
This function has no parameters.
   Returns string on success or null on failure.
  
| Version | Description | 
|---|---|
| 8.0.0 | This method now allows getting the default values' constant names of built-in functions and built-in class methods. Previously, a ReflectionException was thrown. | 
Example #1 Getting default values' constant names of function parameters
<?php
function foo($test, $bar = PHP_INT_MIN)
{
    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->getDefaultValueConstantName() . PHP_EOL;
    }
    echo PHP_EOL;
}
?>
The above example will output:
Name: test Name: bar Default value: PHP_INT_MIN