mail-mock.php

<?php

class MailMockStreamWrapper
{
    private $content = '';
    private $position = 0;

    public function stream_open($path)
    {
        stream_wrapper_restore("file");
        $content = file_get_contents($path);
        $this->content = str_replace("mail", "Mock::mail", $content);
        return true;
    }

    public function stream_read($count)
    {
        $ret = substr($this->content, $this->position, $count);
        $this->position += strlen($ret);
        return $ret;
    }

    public function stream_stat()
    {
        return [];
    }

    public function stream_eof()
    {
        return $this->position >= strlen($this->content);
    }
}

class Mock
{
    public static function mail($to, $subject)
    {
        echo "to => $to, subject => $subject";
    }
}

stream_wrapper_unregister("file");
stream_wrapper_register("file", "MailMockStreamWrapper");

require_once 'mail.php'; // => to => hoge, subject => fuga

mail.php

<?php

mail("hoge", "fuga");