Get single category for a post in different way

id-name-pair.php

//Get a single category name with id, name pair

function get_single_category( $post_id = null ) {
    global $post;
    $post_id = is_null( $post_id ) ? $post->ID : $post_id;
    $categories = wp_get_post_categories( $post_id, array( 'fields' => 'all' ) );
    shuffle( $categories );
    $single_category = array_shift( $categories );

    return array($single_category->term_id => $single_category->name);
}

random.php

// Get a random choice

function get_single_category( $post_id = null ) {
    global $post;
    $post_id = is_null( $post_id ) ? $post->ID : $post_id;
    $categories = wp_get_post_categories( $post_id, array( 'fields' => 'names' ) );
    shuffle( $categories );
    $single_category = array_shift( $categories );

    return $single_category;
}

shift.php

// Get a name, whichever comes first

function get_single_category( $post_id = null ) {
    global $post;
    $post_id = is_null( $post_id ) ? $post->ID : $post_id;
    $categories = wp_get_post_categories( $post_id, array( 'fields' => 'names' ) );
    $single_category = array_shift( $categories );

    return $single_category;
}