I'm not 100% sure what it is you're asking here, but if you'd like to see if a checkbox is set you can do the following:
if(isset($_POST['checkbox-name-here'])){
//it's set code here, maybe set a variable...
$var = "checkbox value";}
That's a rough example, but you should get the drift.
If you're trying to get multiples - then give each checkbox a unique name:
<div id="Oobj58" class="Oobj">
<input type="checkbox" name="sport1" value="Kosz" />koszykówka<br>
<input type="checkbox" name="sport2" value="Zimowe" />sporty zimowe<br>
<input type="checkbox" name="sport3" value="Konie" />jeździectwo konne<br>
</div>
Then in PHP you can check if each value is set:
if(isset($_POST['sport1'])){$sport1="sport1";}
if(isset($_POST['sport2'])){$sport2="sport2";}
if(isset($_POST['sport3'])){$sport3="sport3";}
$email_message .= "Sports: ".clean_string($_POST["sport1"])."\n"
.clean_string($_POST["sport2"])."\n"
.clean_string($_POST["sport3"])."\n";
There are much tidier ways of doing it, but that should help you work it out?
Alternatively you could give each of the checkboxes the same name, and then construct an array out of them. This array can then be looped in the PHP email handler. Look at this SO answer for a great example: https://stackoverflow.com/a/4516887/3112128