The search query string is passed through esc_attr() to ensure that it is safe for placing in an html attribute. This function is used to get the query string when a user performs a search on the site. It returns the query string rather than displaying it, as the the_search_query() function does.
<?php $search_query = get_search_query(); ?>
Examples of the different usage methods.
<?php echo get_search_query(); ?> <?php $search_query = get_search_query(); ?>
get_search_query() is located in