warning: parameter 2 to search_by_title() expected to be a reference, value given inwp-includes\class-wp-hook.php on line 287

#PHP 7.1 doesn’t like & in front of $query parameters in functions.
#If the problem is due a plugin search the & and remove it

#Custom theme (old themes that does not support the new PHP 7.1+ version)
#PHP 7.1 doesn’t like & in front of $query parameters in functions.
#In the Themes -> <your-theme> -> functions.php -> search the code and remove the &
__search_by_title_only( $search, $wp_query ) {