Automatic Internal Links for Keywords

// Keyword => URL (full URL or a path on this site). Examples:
// 'contact us'   => '/contact/',
// 'our services' => '/services/',
$wpide_keyword_links = array();

add_filter( 'the_content', function ( $content ) use ( $wpide_keyword_links ) {
    if ( ! $wpide_keyword_links || ! is_singular() || ! in_the_loop() || ! is_main_query() ) {
        return $content;
    }

    $current = untrailingslashit( (string) get_permalink() );
    $parts   = preg_split( '/(<[^>]+>)/', $content, -1, PREG_SPLIT_DELIM_CAPTURE );
    $depth   = 0;
    $done    = array();
    $tokens  = array();

    foreach ( $parts as $i => $part ) {
        if ( '' === $part ) {
            continue;
        }

        if ( '<' === $part[0] ) {
            if ( preg_match( '/^<(a|h[1-6]|code|pre|script|style|button|textarea)[\s>]/i', $part ) ) {
                $depth++;
            } elseif ( preg_match( '/^<\/(a|h[1-6]|code|pre|script|style|button|textarea)>/i', $part ) ) {
                $depth = max( 0, $depth - 1 );
            }
            continue;
        }

        if ( $depth ) {
            continue;
        }

        foreach ( $wpide_keyword_links as $keyword => $url ) {
            if ( isset( $done[ $keyword ] ) ) {
                continue;
            }

            $href = preg_match( '#^https?://#', $url ) ? $url : home_url( $url );

            if ( untrailingslashit( $href ) === $current ) {
                $done[ $keyword ] = true;
                continue;
            }

            $new = preg_replace_callback( '/\b(' . preg_quote( $keyword, '/' ) . ')\b/iu', function ( $m ) use ( $href, &$tokens ) {
                $tokens[] = '<a class="wpide-auto-link" href="' . esc_url( $href ) . '">' . $m[1] . '</a>';

                return "\x01" . ( count( $tokens ) - 1 ) . "\x02";
            }, $part, 1, $count );

            if ( $count && null !== $new ) {
                $parts[ $i ]      = $new;
                $part             = $new;
                $done[ $keyword ] = true;
            }
        }
    }

    $content = implode( '', $parts );

    foreach ( $tokens as $index => $link ) {
        $content = str_replace( "\x01" . $index . "\x02", $link, $content );
    }

    return $content;
}, 20 );