<?php

$gelen=@$_POST['veri'];
if(empty($gelen)){
?>

<form method="post" action="<?php echo $_SERVER['PHP_SELF'];?>">
<input type="checkbox" name="veri[]" value="5,99"> 1 <br>
<input type="checkbox" name="veri[]" value="14,99"> 2 <br>
<input type="submit" value="Devam Et">
</form>
<?php
}else{
    
    $veri= implode($_POST['veri'],'+');
echo $veri;
     }
?>