Bookingsystem - forhindre dobbeltbooking

På mit bookingsystem har jeg brug for at checke om der allerede er foretaget en booking i samme tidsrum, når brugeren vil tilføje en ny booking.
Eks. hvis der er booket 10.45 - 13.00, skal det ikke kunne lade sig gøre at booke 11.30 - 14.00, men brugeren have en fejlmeddelse.
For nemheds skyld lægger jeg hele scriptet her:

<?php
include_once("core/main.php");

// this is forced here for security reasons

if( !check_module("events2") ) die( "module not enabled" );

require_once("phpdbform/phpdbform_form.php");

$index_page = false;

$page_name = $lang["EVENTS2_ADD"];

draw_header();
$ok_add="N";
if( $cfg["events2"]["useradd1"] )
{
if( $cfg["events2"]["useradd"] )
{
$ok_add="Y";
}
}
if( $cfg["events2"]["useradd"] )
{

if( $_SESSION["wt"]["logged"] )
{ $ok_add="Y"; }

}

if( $ok_add=="N"  )

{

    theme_draw_centerbox_open( $lang["EVENTS2_ADD"] );

    print "<span class=\"error\">{$lang["EVENTS2_NOT_ADD"]}</span>";

    theme_draw_centerbox_close();

    draw_footer();

    exit;


}
else
{

$form = new phpform( "addevent2", "add_event2.php" );

$form->add_static_listbox( "title", $lang["EVENTS2_A_TITLE"], "Bus1, Bus2");

$form->add_date( "date", $lang["EVENTS2_A_DATE"], $cfg["core"]["date_format"]  );

$form->fields["date"]->value = date("Y-m-d");

$form->add_static_listbox( "from_", $lang["EVENTS2_A_FROM"], "00,01,02,03,04,05,06,07,08,09,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,");

$form->add_static_listbox( "from2_", $lang["EVENTS2_A_FROM2"], "00,15,30,45");

$form->add_static_listbox( "till", $lang["EVENTS2_A_TIL"], "00,01,02,03,04,05,06,07,08,09,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24");

$form->add_static_listbox( "still", $lang["EVENTS2_A_STIL"], "00,15,30,45");

$form->add_textbox( "who", $lang["EVENTS2_A_WHO"] , 40, 255 );

$form->add_textarea( "add_desc", $lang["EVENTS2_A_ADDDESC"], 40, 5 );

theme_draw_centerbox_open( $lang["EVENTS2_ADD"] );


$ok = false;

$errors = array();

if( $form->process() )

{

    if( !isset($_SESSION["wt"]["addevent2"]) )

    {



        $form->fields["title"]->value = trim(strip_tags($form->fields["title"]->value));   
        $form->fields["date"]->value = trim(strip_tags($form->fields["date"]->value));
        $form->fields["from_"]->value = trim(strip_tags($form->fields["from_"]->value));
        $form->fields["from2_"]->value = trim(strip_tags($form->fields["from2_"]->value));
        $form->fields["till"]->value = trim(strip_tags($form->fields["till"]->value));
        $form->fields["still"]->value = trim(strip_tags($form->fields["still"]->value));   

        $form->fields["who"]->value = trim(strip_tags($form->fields["who"]->value));
        $form->fields["add_desc"]->value = trim(strip_tags($form->fields["add_desc"]->value));
        $form->fields["location"]->value = trim(strip_tags($form->fields["location"]->value));

        if( !empty($errors) )

        {

            print "<div class=\"error\">";

            while( $err = each($errors) )

            {
                print "{$err[1]}<br>\n";

            }

            print "</div>\n";
   

        } else $ok = true;


        if( $ok )

        {

            $stmt = "insert into {$config["prefix"]}_events2 set"

                ." title = '".addslashes($form->fields["title"]->value)."',"
               
                ." date = '".addslashes($form->fields["date"]->value)."',"

                ." active = 'Y',"

               
                ." from_ = '".addslashes($form->fields["from_"]->value)."',"
               
                ." from2_ = '".addslashes($form->fields["from2_"]->value)."',"
               
                ." till = '".addslashes($form->fields["till"]->value)."',"

                ." still = '".addslashes($form->fields["still"]->value)."',"
               
                ." who = '".addslashes($form->fields["who"]->value)."',"

                ." userid ='".$_SESSION[wt][uid]."',"


                ." add_desc = '".addslashes($form->fields["add_desc"]->value)."',"

                ." location = '".addslashes($form->fields["location"]->value)."'";

            $ret = db_query( $stmt );

            if( !$ret )

            {

                $ok = false;

                print "<div class=\"error\">".$lang["ERROR_04"]."<br>".db_error()."</div>";

            } else {
if( $cfg["events2"]["sendmail"] )
{
                print "<div class=\"info\">".$lang[EVENTS2_ADD_OK]."</div>";
                mail($cfg["core"]["mail_admin"], $lang["EVENTS2_M_01"], $lang["EVENTS2_M_02"]."\n\n".$cfg["core"]["url"],"From: ".$cfg["core"]["mail_admin"]."\n");
}
                $_SESSION["wt"]["addevent2"] = true;

            }

        }

    } else $ok = true;

} else {

    if( isset($_SESSION["wt"]["addevent2"]) ) unset($_SESSION["wt"]["addevent2"]);

}

if( !$ok ) $form->draw();

theme_draw_centerbox_close();


theme_draw_centerbox_open( $lang["EVENTS_LIST_EVENTS"] );
events2_draw_list();

theme_draw_centerbox_close();

draw_footer();
}
?>


Jeg vil gerne checke disse: title, from_, from2_, till, still.