<?php
if(isset($_POST['submit']))
{
    if(isset($_POST['selection']) && is_array($_POST['selection']))
    {
        if(count((array) $_POST['selection']) == 1)
        {
            $query_build = '= ' . $_POST['selection'][0];
        }
        else
        {
            $query_build = 'IN (' . implode(', ', $_POST['selection']) . ')';
        }

        //$query = mysql_query("DELETE FROM table WHERE id $query_build");
        echo "DELETE FROM table WHERE id $query_build";
    }
    else
    {
        echo 'You must select 1';
    }
}
?>

<form action="" method="post">
    <input type="checkbox" name="selection[]" value="1" /> 1
    <input type="checkbox" name="selection[]" value="2" /> 2
    <input type="checkbox" name="selection[]" value="3" /> 3
    <input type="submit" name="submit" value="delete" />
</form>