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ReflectionParameter::isOptional is a method in PHP's ReflectionParameter class that is used to determine if a function or method parameter has a default value.

When using reflection in PHP, you can inspect the parameters of a function or method using the ReflectionParameter class. The isOptional method can then be called on an instance of ReflectionParameter to check if the parameter has a default value.

Here's an example to demonstrate its usage:

php
function foo($param1, $param2 = 'default') {
// some code
}

$reflectionFunc = new ReflectionFunction('foo');

$parameters = $reflectionFunc->getParameters();

foreach ($parameters as $parameter) {
if ($parameter->isOptional()) {
echo $parameter->getName() . ' is optional';
} else {
echo $parameter->getName() . ' is required';
}
}


In this example, the isOptional method is used to determine if the $param2 parameter in the foo function has a default value. If it does, the method will output 'param2 is optional', otherwise it will output 'param2 is required'.