How to remove custom post type slug from url or permalink ?

First, we will remove the slug from the permalink:

function na_remove_slug( $post_link, $post, $leavename ) {

    if ( 'events' != $post->post_type || 'publish' != $post->post_status ) {
        return $post_link;
    }

    $post_link = str_replace( '/' . $post->post_type . '/', '/', $post_link );

    return $post_link;
}
add_filter( 'post_type_link', 'na_remove_slug', 10, 3 );

Just removing the slug isn't enough. Right now, you'll get a 404 page because WordPress only expects posts and pages to behave this way. You'll also need to add the following:

function na_parse_request( $query ) {

    if ( ! $query->is_main_query() || 2 != count( $query->query ) || ! isset( $query->query['page'] ) ) {
        return;
    }

    if ( ! empty( $query->query['name'] ) ) {
        $query->set( 'post_type', array( 'post', 'events', 'page' ) );
    }
}
add_action( 'pre_get_posts', 'na_parse_request' );

Just change "events" to your custom post type and you're good to go. You may need to refresh your permalinks.

How to get my domain or server url with php?

<?php
$protocol = strpos(strtolower($_SERVER['SERVER_PROTOCOL']),'https')=== FALSE ? 'http' : 'https';
$host = $_SERVER['HTTP_HOST'];
$website = $protocol . '://' . $host;
?>

<a href="<?php echo $website; ?>">my site</a>

How to use $_SERVER['SERVER_NAME'] with a href ?

<?php
$protocol = strpos(strtolower($_SERVER['SERVER_PROTOCOL']),'https')=== FALSE ? 'http' : 'https';
$host = $_SERVER['HTTP_HOST'];
$website = $protocol . '://' . $host;
?>

<a href="<?php echo $website; ?>">my site</a>

PHP $_SERVER['SERVER_NAME'] correct use

<?php
$protocol = strpos(strtolower($_SERVER['SERVER_PROTOCOL']),'https')=== FALSE ? 'http' : 'https';
$host = $_SERVER['HTTP_HOST'];
$website = $protocol . '://' . $host;
?>

<a href="<?php echo $website; ?>">my site</a>

How to get categories in wordpress?

Dropdown Box

Here's how to create a dropdown box of the subcategories of, say, a category that archives information on past events. This mirrors the example of the dropdown example of wp_get_archives which shows how to create a dropdown box for monthly archives.

Suppose the category whose subcategories you want to show is category 10, and that its category "nicename" is "archives".

<select name="event-dropdown" onchange='document.location.href=this.options[this.selectedIndex].value;'> 
 <option value=""><?php echo esc_attr(__('Select Event')); ?></option> 
 <?php 
  $categories = get_categories('child_of=10'); 
  foreach ($categories as $category) {
   $option = '<option value="/category/archives/'.$category->category_nicename.'">';
 $option .= $category->cat_name;
 $option .= ' ('.$category->category_count.')';
 $option .= '</option>';
 echo $option;
  }
 ?>
</select>

List Categories and Descriptions

This example will list in alphabetic order, all categories presented as links to the corresponding category archive. Each category descripition is listed after the category link.

<?php
$args = array(
  'orderby' => 'name',
  'order' => 'ASC'
  );
$categories = get_categories($args);
  foreach($categories as $category) { 
    echo '<p>Category: <a href="' . get_category_link( $category->term_id ) . '" title="' . sprintf( __( "View all posts in %s" ), $category->name ) . '" ' . '>' . $category->name.'</a> </p> ';
    echo '<p> Description:'. $category->description . '</p>';
    echo '<p> Post Count: '. $category->count . '</p>';  } 
?>

Get only top level categories

To get the top level categories only, set parent value to zero. This example gets link and name of top level categories.

<?php
$args = array(
  'orderby' => 'name',
  'parent' => 0
  );
$categories = get_categories( $args );
foreach ( $categories as $category ) {
 echo '<a href="' . get_category_link( $category->term_id ) . '">' . $category->name . '</a><br/>';
}
?>

Source: https://codex.wordpress.org/Function_Reference/get_the_category_list