How to highlight search terms without plugin

Add these 2 functions to your functions.php

function search_excerpt_highlight() {
    $excerpt = get_the_excerpt();
    $keys = implode('|', explode(' ', get_search_query()));
    $excerpt = preg_replace('/(' . $keys .')/iu', '<strong class="search-highlight">\0</strong>', $excerpt);

    echo '<p>' . $excerpt . '</p>';
}

function search_title_highlight() {
    $title = get_the_title();
    $keys = implode('|', explode(' ', get_search_query()));
    $title = preg_replace('/(' . $keys .')/iu', '<strong class="search-highlight">\0</strong>', $title);

    echo $title;
}

Edit:

To use the_content for your search results use the function below:

function search_content_highlight() {
        $content = get_the_content();
        $keys = implode('|', explode(' ', get_search_query()));
        $content = preg_replace('/(' . $keys .')/iu', '<strong class="search-highlight">\0</strong>', $content);

        echo '<p>' . $content . '</p>';
    }

In your loop or search.php file call <?php search_title_highlight(); ?> instead of <?php the_title(); ?> and use <?php search_excerpt_highlight(); ?> instead of <?php the_excerpt(); ?>

In your css add the search-highlight class which will highlight all searched words in yellow.

.search-highlight {
    background:#FFFF00  
    }