How to make my new theme read [youtube id="id of the video here" width="600" height="350"]?

You can for example:

Copy the YouTube shortcode from the old theme to your new theme.

Locate the line (most likely in the functions.php file):
```
add_shortcode( 'youtube', 'some_function' );
```
and copy this line and the some_function() to your functions.php in your new
theme or better yet – create a new plugin file including:
```
/*Plugin Name: YouTube embed shortcode from old theme */

add_shortcode( 'youtube', 'some_function' );

some_function( $args = array(), $content=""){ /* ...code... */}
```
Add a YouTube embed plugin with the same parameters.

Create your own plugin to handle this.

Here’s a poor man’s version (untested):

/*Plugin Name: YouTube embed shortcode - poor man's version */

add_shortcode( 'youtube', 'ytb_sc' );

if( ! function_exists( 'ytb_sc' ) ):

    function ytb_sc( $atts = array(), $content="" )
    {
       $atts = shortcode_atts( array(
           'id'      => '',             // default id
           'width'   => 640,            // default width (px)
           'height'  => 480,            // default height (px)
       ), $atts, 'ytb_sc' );

         // Sanitize input:
         $id      = esc_attr( $atts['id'] );
         $width   = (int) $atts['width'];
         $height  = (int) $atts['height'];

        if( ! empty( $id ) ) {
            $sc = sprintf( '%s%s',
                           $width,
                           $height,
                           'https://www.youtube.com/watch?v=',
                           $id );
        } else {
            return __( 'Missing YouTube ID!' );
        }

        // Output
        return $GLOBALS['wp_embed']->run_shortcode($sc);
    }

endif;

Hope this helps!

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