Automatic Internal Links for Key Phrases

// Phrase => URL, and the maximum number of automatic links per post.
$wpide_auto_links = array(
    'technical SEO'   => home_url( '/services/technical-seo/' ),
    'content audit'   => home_url( '/guides/content-audit/' ),
    'local business'  => home_url( '/local-seo/' ),
);
$wpide_auto_links_max = 5;

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

    $current = untrailingslashit( get_permalink() );
    $parts   = preg_split( '/(<[^>]+>)/', $content, -1, PREG_SPLIT_DELIM_CAPTURE );
    $skip    = 0;
    $used    = array();
    $count   = 0;

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

        if ( '<' === $part[0] ) {
            // Track whether we are inside an element whose text must not be linked.
            if ( preg_match( '#^<(a|h[1-6]|code|pre|button|script|style|textarea|figcaption)\b#i', $part ) ) {
                $skip++;
            } elseif ( preg_match( '#^</(a|h[1-6]|code|pre|button|script|style|textarea|figcaption)>#i', $part ) ) {
                $skip = max( 0, $skip - 1 );
            }
            continue;
        }

        if ( $skip || $count >= $wpide_auto_links_max ) {
            continue;
        }

        foreach ( $wpide_auto_links as $phrase => $url ) {
            if ( isset( $used[ $phrase ] ) || $count >= $wpide_auto_links_max || untrailingslashit( $url ) === $current ) {
                continue;
            }

            $pattern = '/(?<![\p{L}\p{N}])(' . preg_quote( $phrase, '/' ) . ')(?![\p{L}\p{N}])/iu';

            if ( preg_match( $pattern, $part ) ) {
                $part = preg_replace( $pattern, '<a href="' . esc_url( $url ) . '" class="auto-link">$1</a>', $part, 1 );
                $used[ $phrase ] = true;
                $count++;
            }
        }

        $parts[ $i ] = $part;
    }

    return implode( '', $parts );
}, 20 );