搜索引擎，当用户想要“全部”时该怎么做

I'm an absolute noob to programming and over the past 10 days or so I have been building a 'buy sell trade' website for things.

Anyway I have got to the point where I now have the ability for people to register, post ads and reply to them... but now I want to have a page where they can search through the adverts.

The search form works on 3 fields, 'make', 'model' and 'caliber' (site is for guns)

    <form action="" method="get" autocomplete="off">
I'm looking for a 
<input type="text" name="make" class="autosuggestmake" placeholder="Manufacturer"/>
        <div class="dropdown">
            <ul class="resultmake"></ul>
        </div>

<input type="text" name="model" class="autosuggestmodel" placeholder="Model"/>
        <div class="dropdown">
            <ul class="resultmodel"></ul>
        </div>
in
<select name="caliber" >
    <option value="*">(Caliber) Any</option>
    <option value=".177">.177</option>
    <option value=".20">.20</option>
    <option value=".22">.22</option>
    <option value=".25">.25</option>
</select>
<input type="submit" value="Search" />

This is posted as GET data which I 'catch' with this code:

    $advert = new Advert;
    if (empty($_GET) === true){
$adverts = $advert->fetch_all();
    } else {
$search_make = $_GET['make'];
$search_model = $_GET['model'];
$search_caliber = $_GET['caliber'];

$adverts = $advert->fetch_results($search_make, $search_model, $search_caliber);
    }

My fetch_results query is this:

    class Advert {
public function fetch_results($search_make, $search_model, $search_caliber) {
    global $pdo;

    $search_caliber = mysql_real_escape_string($search_caliber);
    $search_make = mysql_real_escape_string($search_make);
    $search_model = mysql_real_escape_string($search_model);

    if (empty($search_make) === true){
        $search_make = "*";
    } 

    if (empty($search_model) === true){
        $search_model = "*";
    } 

    $query = $pdo -> prepare("SELECT * FROM `adverts` WHERE make = '$search_make' AND model = '$search_model' AND caliber = '$search_caliber'");
    $query -> execute();

    return $query -> fetchAll();
}

On my last question someone told me to start using PDO, so I did :)

My problem is when someone fills in the make field on my form and nothing else it will return nothing. I thought if the get variables were blank I would append a * and it will return anything but this is not the case :( I've been searching but I think my problem is I don't know the correct words to search to find the cure for my problem...

Any help would be greatly appreciated.

Having implemented a site with this myself, I strongly recommend Sphinx. This is what craigslist uses. It is simple enough for your needs, yet powerful enough to grow with you.

Another alternative is ElasticSearch, which I'm told is very good as well.

There are two approaches you can take. One is to rewrite the query in php based on the search conditions.

The other is a SQL solution:

SELECT *
FROM `adverts`
WHERE ('$search_make' = '' or make = '$search_make') AND
      ('$search_model' or model = '$search_model') AND
      ('$search_caliber' or caliber = '$search_caliber');

However, if the table has indexes on the columns, then this query probably will not use the indexes. Writing the php to include only the clauses with values creates a more efficient query.

Either you let your users browse "all" -- Use case = some collector looking for odd items.

Or you have a "Search By" pull down and make your users choose at least one of the search categories.

You could just send an error message if all the search criteria are blank -- but this tends to annoy users more than leading them through a set of search option screens.