How to write unit test for code calling a class from a library with static variable and constructor?

I am writing some unit test for some legacy code as a preparation for some refactoring. One section of the code is calling a class from a library where I cannot change the code in it. The code is using its constructor and calling some static variable. May I ask how can I mock the method for testing? (or how do I test the method without mocking?)

I have tried to simplify my code for an example:

<?php
//This is from the library code that I am unable to change
class ResponseMessage {
    const STATUS_ERROR = 500;
    const STATUS_SUCCESS = 200;

    function __construct( $responseMessage, $responseCode ) {
        $this->responseMessage = $responseMessage;
        $this->responseCode = $responseCode
    }
    
    public function sendMessage($sendToAdmin = false) {
        // Some code to send email and trigger some API to notice other service, so this needed to be mocked in unit test to stop it. It is also needed to check if this function is actually called.
    }
    
    //.... Some other methods and variables
}

///////////////////////////////////////////////////////////////////
//This is my class to be tested. If possible I would like to avoid changing any code, but if it is necessary this code can be changed a bit for injecting mock class.
class myClass {
    // ... some other methods where I have no problem testing....
    function handle($input) {
        //The actual comparision is much more complicated. Simplified for this question
        if (empty($input)) {
            // Some other error handling, no response message is generated
            return;
        }
        if ( $input == 1 )
            $responseMessage = new ResponseMessage('The input is 1', ResponseMessage::STATUS_SUCCESS );
        else if ( $input == 2 ）
            $responseMessage = new ResponseMessage('The input is 2', ResponseMessage::STATUS_SUCCESS )；
        else
            $responseMessage = new ResponseMessage('Unexpected input', ResponseMessage::STATUS_ERROR )；
        
        $respnoseMessage->sendMessage();

        $responseMessageToAdmin = new ResponseMessage('The comaparison is triggered', RespnoseMessage::STATUS_SUCCESS);
        $responseMessageToAdmin->sendMessage(true);
    }
}
///////////////////////////////////////////////////////////////////
//This is my testing class in PHPUnit.
class testMyClass {
    public function testProcessWithInputOne {
        // how should I mock ResponseMessage here?
        $myClass = new MyClass();
        $myClass->handle(1);
    }
}