Higher PHP version causes site to break

Hi Alvind
Here you go.
Cheers Jodie

<?php
/**
 * GeneratePress child theme functions and definitions.
 *
 * Add your custom PHP in this file. 
 * Only edit this file if you have direct access to it on your server (to fix errors if they happen).
 */

function generatepress_child_enqueue_scripts() {
	if ( is_rtl() ) {
		wp_enqueue_style( 'generatepress-rtl', trailingslashit( get_template_directory_uri() ) . 'rtl.css' );
	}
}
add_action( 'wp_enqueue_scripts', 'generatepress_child_enqueue_scripts', 100 );
get_avatar( get_the_author_meta( $author_id ));

function ta_add_noindex_headers() {
    header( 'X-Robots-Tag: noindex, nofollow' );
}
 
add_action( 'ta_before_link_redirect' , 'ta_add_noindex_headers' );

add_action( 'after_setup_theme', function() {
    add_theme_support( 'wp-block-styles' );
} );

add_filter( 'generate_post_date_output', function( $output, $time_string ) {
	printf( '<span class="posted-on">%s</span>',
		$time_string
	);
}, 10, 2 );

add_shortcode('portable_hook', function($atts){
	ob_start();
        $atts = shortcode_atts( array(
            'hook2_name' => 'no foo'
        ), $atts, 'portable_hook' );
		do_action($atts['hook2_name']);
	return ob_get_clean();
});

add_filter( 'the_content', 'insert_featured_image_with_meta_check', 20 );

function insert_featured_image_with_meta_check( $content ) {
	global $post;
    
	$inserted_hook = do_shortcode('[portable_hook hook2_name="after_first_image"]');
	return prefix_insert_after_first_image( $inserted_hook, 1, $content );
}

function prefix_insert_after_first_image( $insertion, $image_id, $content ) {
	$images = preg_split( '/(<img[^>]+\>)/i', $content, -1, PREG_SPLIT_DELIM_CAPTURE );
 
	foreach ( $images as $index => $image ) {
    	if ( $image_id == $index ) {
        	$images[$index] .= $insertion;
    	}
	}
 
	return implode( '', $images );
}

function gravatar_alt($text) {
    $alt = get_the_author_meta( 'display_name' );
    $text = str_replace('alt=\'\'', 'alt=\'Avatar for '.$alt.'\' title=\'Gravatar for '.$alt.'\'',$text);
    return $text;
}
add_filter('get_avatar','gravatar_alt');

add_filter( 'generate_hook_element_display', function( $display, $element_id ) {
	if(400138 === $element_id && !is_admin()) {
		$category = get_category( get_query_var( 'cat' ) );
		var_dump($category->category_parent);
		if($category->category_parent > 0) {
			return false;
		}
	} 
	return $display;
    
}, 10, 2 );

add_filter( 'generate_comments_title_output', function( $output, $comments_title ) {
    return sprintf(
        '<p class="comments-title">%s</p>',
        esc_html( $comments_title )
    );
}, 10, 2 );

add_filter( 'comment_form_defaults', 'custom_reply_title' );
function custom_reply_title( $defaults ){
  $defaults['title_reply_before'] = '<p id="reply-title" class="comment-reply-title">';
  $defaults['title_reply_after'] = '</p>';
  return $defaults;
}

add_filter( 'generate_comments_title_output', function( $output, $comments_title ) {
    $comments_title = 'Comments';
    return sprintf(
        '<p class="comments-title">%s</p>',
        esc_html( $comments_title )
    );
}, 10, 2 );

add_filter( 'generate_element_display', function( $display, $element_id ) {
    if ( 1533194 === $element_id && is_author( 'jodie-morgan' ) ) {
        $display = true;
    }

    return $display;
}, 10, 2 );

add_filter('render_block', function ($block_content, $block) {
	if (!is_admin() && ! empty($block['attrs']['className']) && strpos($block['attrs']['className'], 'add-nofollow') !== false) {
		$robots_meta = get_post_meta(get_the_ID(), 'rank_math_robots', true);

		if ( strpos($robots_meta, 'noindex') !== false ) {
			$myreplace = 'href';
			$myinsert = 'rel="nofollow" href';
			$block_content = str_replace($myreplace, $myinsert, $block_content);
		}
	}

	return $block_content;
}, 10, 2);

// links with class important-link
// open in new window and rel no follow
add_filter( 'render_block', function( $block_content, $block ) {
    // check the block for class names
    if ( 
        ! empty( $block['attrs']['className'] ) 
        && 'important-link' === $block['attrs']['className']
    ) {
        $block_content = str_replace( '<a ', '<a target="_blank" rel="nofollow" ', $block_content );
    }

    return $block_content;
}, 10, 2 );

add_filter( 'generate_404_text','generate_custom_404_text' );
function generate_custom_404_text()
{
    return '';
}
add_filter( 'get_search_form','my_remove_search_form' );
function my_remove_search_form()
{
    $template = $GLOBALS['template'];
    $template_file = substr($template, strrpos($template, '/') + 1);
    if ($template_file === '404.php') {
        return '';
    }
}

add_filter( 'generate_404_title','remove_404_title' );
function remove_404_title()
{
      return '';
}

function disable_comments_rss_feed( $for_comments ) {
	if ( is_comment_feed() ) {
    	return '';
	}
	return $for_comments;
}
add_filter( 'commentsrss2_comments_feed', 'disable_comments_rss_feed' );

add_action( 'after_setup_theme', 'tu_add_comment_url_filter' );
function tu_add_comment_url_filter() {
	add_filter( 'comment_form_default_fields', 'tu_disable_comment_url', 20 );
}

function tu_disable_comment_url($fields) {
	unset($fields['url']);
	return $fields;
}

add_action( 'template_redirect', 'noindex_feeds' );
function noindex_feeds() {
    if ( ! is_feed() ) {
        return;
    }
header( 'X-Robots-Tag: noindex', true );
	}

Thanks very much Alvind. So do I replace that first and then try to update the PHP version to 8 to see if the site functions as normal?

Hi Alvind. I’ve done as you’ve instructed and the site has been upgraded to PHP Version 8.3 and it’s working now.
Thank you very much for your help.
Cheers Jodie