http_init();
//Call add.php and overgive a string s with the content "Blabla"
http_post(_str("http://www.host.com/add.php"),_str("s=Blabla"));

<?php
//Get the content of 's':
string_s = _POST['s'];
?>

<?
mysql_connect("server_adress","user,"pw") or die("error, could not connect to mysql-server");
$search=$_GET["s"]; // get the searched query (if not, try $_POST)
$row_name="row"; // insert the row name, where to search
$table_name="table"; // Insert the table name
$link=mysql_query("SELECT * FROM $table_name WHERE $row_name LIKE '$search'");
if($link && $a=mysql_fetch_object($link)) // found!
echo "1"; // Give out 1, means the string is found
else
{
mysql_query("INSERT INTO $table_name VALUES ...");
echo "0"; // Give out 0, means the string was not found but was insert now
}
?>

mysql_query("DELETE FROM $table_name WHERE $row_name LIKE '$search'");