Shortcode is a small piece of code that can be added into the WordPress editor and will output something different once the page is published or previewed.

Frequently, shortcodes are added to the theme functions.php file, but that's not a good practice as shortcodes are expected to keep working after changing themes. Instead, write a plugin to add this functionality.

The structure for registering shortcode is:

function new_shortcode($atts, $content = null){
    // if parameters are needed in the shortcode
    // parameters can be set to default to something
    extract( shortcode_atts( array(
        'param_one' => 'h1'
    ), $atts ) );
    $shortcode = '<'.$param_one'>'.$content.'</'.$param_one.'>';
    return $shortcode;
}
// this is what registers the shortcode with wordpress
add_shortcode('demo-shortcode','new_shortcode');

Inside the WordPress editor, you can type:

[demo-shortcode param_one="h2"]Demo[/demo-shortcode]
// you don't need to insert param_one into the editor if it has a default value.
// having it in the editor will override the default

Once the page is published, this will turn into

<h2>Demo</h2>

[footag foo="value of 1" attribute-2="value of 2"]

In wordpress admin we use pre defined shortcodes by writing the shortcode name inside square brackets and optionally adding attributes to it separating by space.

function footag_func( $atts ) {
    return "foo = {$atts['foo']}";
}
add_shortcode( 'footag', 'footag_func' );

In plugins we can add shortcodes using the add_shortcode function.

The shortcode can be used in any Wordpress page or post just by enclosing it in square brackets.

[footag]

<?php echo do_shortcode("[footag foo='Hi! I am a foo output']"); ?>

To print a shortcode using php use the do_shortcode function and echo the returned value.

add_filter( 'widget_text', 'shortcode_unautop' );
add_filter( 'widget_text', 'do_shortcode' );enter code here

Add this to a plugin or the functions.php file to enable shortcodes in widgets. The code first stops WordPress turning line breaks into paragraph tags and then lets shortcodes to parse for widgets. The order of the two lines is important.