if(isset($_POST['Delete']) && !isset($_POST['Yes']) && !isset($_POST['No'])){  
    //so form has been submitted, but we've not yet confirmed
    //Print confirmation form, which has all the previous values hidden:

    //Do checks first on inputs
    $del_id = $_POST['del_id'];

    echo 'Are you sure you wish to delete record number '. $del_id .'?<br />';
    echo '<form target="'. $_SERVER['PHP_SELF'] .'" method="post">'.
           '<input type="hidden" name="del_id" value="'. $del_id .'">'.
           '<input type="hidden" name="Delete">'.
           '<input type="submit" name="Yes"> <input type="submit" name="No">';

} 
elseif(isset($_POST['Delete']) && isset($_POST['Yes'])){
    //Form has been submitted & confirmed!

    delete($_POST['del_id']); //your function
}
elseif(isset($_POST['Delete']) && isset($_POST['No'])){
    //Deletion was cencelled
    echo 'Deletion cancelled';
}
else{  //default, print form asking for recored number to be deleted

    echo 'Please enter the record ID you wish to delete<br />';
    echo '<form target="'. $_SERVER['PHP_SELF'] .'" method="post">'.
           '<input type="text" name="del_id">'.
           '<input type="submit" name="Delete">';
}