1.php

<?php
class Something
{
    public function receiveData($data)
    {
        // do something.

        mail($to, $subject, $message);

        // do something.
    }
}

2.php

<?php
class Something
{
    public function receiveData($data)
    {
        // do something.

        $this->mail($to, $subject, $message);

        // do something.
    }

    protected function mail($to, $subject, $message)
    {
        mail($to, $subject, $message);
    }
}

class FakeSomething extends Something
{
    protected function mail($to, $subject, $message)
    {
    }
}

class SomethingTest extends PHPUnit_Framework_TestCase
{
    public function testReceiveData()
    {
        $something = new FakeSomething();

        // write tests
    }
}

3.php

<?php
class Something
{
    private $mailFunction;
    public function __construct()
    {
        $this->mailFunction = function () {
            call_user_func_array('mail', func_get_args());
        };
    }

    public function receiveData($data)
    {
        // do something.

        call_user_func_array($this->mailFunction, array($to, $subject, $message));

        // do something.
    }

    public function setMailFunction(callable $func)
    {
        $this->mailFunction = $func;
    }
}

class SomethingTest extends PHPUnit_Framework_TestCase
{
    public function testReceiveData()
    {
        $something = new Something();
        $something->setMailFunction(function () {
            // dummy
            return;
        });

        // write tests
    }
}

4.php

<?php
class SomethingTest extends PHPUnit_Framework_TestCase
{
    public function testReceiveData()
    {
        $something = new Something();
        
        $something->setMailFunction(function ($to, $subject, $message) {
            $this->assertSame('[email protected]', $to);
            $this->assertSame('sample subject', $subject);
            $this->assertSame('hello', $message);
            return;
        });

        $something->receiveData('data');

        // write tests
    }
}