Drupal menu system callback mapping

How does drupal menu system callback mapping happen?

When a user makes a request to drupal through a browser, using the request URL drupal menu system decides which callback function to invoke.

what is a callback function?

Callback function in this case is a php function which gets called when a user trying to access a path that has already been registered in a menu item.

In the given example-1 if a browser makes a request using the URL "http://example.com/hello", menu system will look for a menu item which contains that path and invokes the relevant callback function in this case, "hello()".

example-1

<?php

//hook_perm implementation  
function example_perm() {
    return array(
        'access hello'
    );
}

//hook_menu implementation  
function example_menu() {
    $items = array();
    //menu item   
    $items['hello'] = array(
    'title' =--> t('hello'
    ), 'page callback' => 'hello', 'page arguments' => array(
    0
    ), 'access arguments' => array(
    'access hello'
    ), 'type' => MENU_CALLBACK);

    return $items;
}

//callback function  
function hello() {
    echo "hello!";
}
?>  

Output for example-1

hello!

example-2

This example shows how to get city data in json or xml using a request URL.

<?php

//hook_perm implementation
function example_perm() {
    return array(
        'access hello'
    );
}

//hook_menu implementation
function example_menu() {
    $items = array();
    //menu item 
    $items['hello'] = array(
        'title' => t('hello'
        ), 'page callback' => 'hello', 'page arguments' => array(
            0
        ), 'access arguments' => array(
            'access hello'
        ), 'type' => MENU_CALLBACK);

    $items['full_city_list_json'] = array(
        'title' => t('City List JSON'
        ), 'page callback' => 'full_city_list_json', 'page arguments' => array(
            0
        ), 'access arguments' => array(
            'access hello'
        ), 'type' => MENU_CALLBACK);

    $items['full_city_list_xml'] = array(
        'title' => t('City List XML'
        ), 'page callback' => 'full_city_list_xml', 'page arguments' => array(
            0
        ), 'access arguments' => array(
            'access hello'
        ), 'type' => MENU_CALLBACK);

    return $items;
}

function hello() {
    echo "hello!";
}

function full_city_list_json() {
    $response = array();
    $result = db_query("SELECT  tid,name FROM term_data  WHERE  vid=2 order by name ASC");
    while ($city_data = db_fetch_object($result)) {
        $response[$city_data->name]['city_name'] = $city_data->name;
        $response[$city_data->name]['tid'] = $city_data->tid;
    }

    if (!empty($response)) {
        //encodes city data as json  
        $jsonstrCityList = json_encode($response);
        echo $jsonstrCityList;
    } else {
        return false;
    }
}

function full_city_list_xml() {
//sending city data as xml
    $dom = new DOMDocument("1.0");
    $root_elm = $dom->createElement("markers");
    $root = $dom->appendChild($root_elm);

    $result = db_query("SELECT  tid,name FROM term_data  WHERE  vid=2 order by name ASC");

    while ($city_data = db_fetch_object($result)) {

        $marker_elm = $dom->createElement("marker");
        $marker_node = $root->appendChild($marker_elm);

        $marker_node->setAttribute("name", $city_data->name);
        $marker_node->setAttribute("tid", $city_data->tid);
    }
    header("Content-type: text/xml");
    echo $dom->saveXML();
}

?>

Outputs for example-2

for http://example.com/full_city_list_json

{"Agalawatta":{"city_name":"Agalawatta","tid":"156"},"Ahungalla":{"city_name":"Ahungalla","tid":"214"},"Akurana":{"city_name":"Akurana","tid":"164"},"Akuressa":{"city_name":"Akuressa","tid":"162"},"Aluthgama":{"city_name":"Aluthgama","tid":"120"},"Ambalangoda":{"city_name":"Ambalangoda","tid":"125"},"Ampara":{"city_name":"Ampara","tid":"211"},"Anuradhapura":{"city_name":"Anuradhapura","tid":"46"},"Arugambay":{"city_name":"Arugambay","tid":"114"},"Avissawella":{"city_name":"Avissawella","tid":"212"},"Badulla":{"city_name":"Badulla","tid":"108"},"Balangoda":{"city_name":"Balangoda","tid":"149"},"Balapitiya":{"city_name":"Balapitiya","tid":"486"},"Bandarawela":{"city_name":"Bandarawela","tid":"197"}}

for http://example.com/full_city_list_xml

<markers>
<marker name="Agalawatta" tid="156"/>
<marker name="Ahungalla" tid="214"/>
<marker name="Akurana" tid="164"/>
<marker name="Akuressa" tid="162"/>
<marker name="Aluthgama" tid="120"/>
<marker name="Ambalangoda" tid="125"/>
<marker name="Ampara" tid="211"/>
<marker name="Anuradhapura" tid="46"/>
<marker name="Arugambay" tid="114"/>
<marker name="Avissawella" tid="212"/>
<marker name="Badulla" tid="108"/>
<marker name="Balangoda" tid="149"/>
<marker name="Balapitiya" tid="486"/>
<marker name="Bandarawela" tid="197"/>
</markers>

As in example-2 after you grabbed the city data in json or in xml you can parse them and use in any of your applications.