In fact, SMThemes is a WordPress theme. See the comment at the end of the link.
get_header()
is defined in the WordPress core itself and not in the theme files, due this you weren't able to find it there.
You can find its definition at wp-installation-root/wp-includes/general-template.php
file, Line 24, and its definition is:
/**
* Load header template.
*
* Includes the header template for a theme or if a name is specified then a
* specialised header will be included.
*
* For the parameter, if the file is called "header-special.php" then specify
* "special".
*
* @uses locate_template()
* @since 1.5.0
* @uses do_action() Calls 'get_header' action.
*
* @param string $name The name of the specialised header.
*/
function get_header( $name = null ) {
do_action( 'get_header', $name );
$templates = array();
$name = (string) $name;
if ( '' !== $name )
$templates[] = "header-{$name}.php";
$templates[] = 'header.php';
// Backward compat code will be removed in a future release
if ('' == locate_template($templates, true))
load_template( ABSPATH . WPINC . '/theme-compat/header.php');
}
As of your real question, you can follow @SteAp suggestion and give PHPStorm a try, its very nice indeed and this kind of issues you will solve them easily (I don't work for JetBrains).