How to debug function file causing white screen

Question

My function.php file seems to be causing a white screen to appear whenever I try to login to the dashboard locally. I have confirmed this by renaming the 'function.php' file and also the plugins file to identify the problem area.

I'm pretty new to Wordpress and would like to know how to exactly pinpoint the line of code in the function.php file thats causing problems. Below is my code.

Prior to this I also noticed that my media library would not work. The spinner would simply spin and show none of my images

function.php

<?php

// Call Stylesheets
function wpSam_theme_styles(){

   wp_enqueue_style( 'normalize_css', get_template_directory_uri() . '/css/normalize.min.css' );
   wp_enqueue_style( 'main_css', get_template_directory_uri() . '/style.css' );

}
add_action('wp_enqueue_scripts','wpSam_theme_styles');


// Call Scripts
function wpSam_theme_js(){

// 1.name 2. 3. Directory 4. dependencies  5. 6. appear in footer (t/f)
wp_enqueue_script( 'modernizer_js', get_template_directory_uri() . '/js/modernizr.js' , '', '', false );
wp_enqueue_script( 'main_js', get_template_directory_uri() . '/js/app.js' , '', '', true );
}
add_action('wp_enqueue_scripts', 'wpSam_theme_js');


function time_ago( $type = 'post' ) {
$d = 'comment' == $type ? 'get_comment_time' : 'get_post_time';

return human_time_diff($d('U'), current_time('timestamp')) . " " . __('ago');

 }

add_theme_support( 'post-thumbnails' ); 

add_action( 'after_setup_theme', 'wpse239844_image_sizes' );

 function wpse239844_image_sizes() {
    set_post_thumbnail_size( 470, 295, true ); // $width, $height, $crop
 }


  add_action( 'post_stuck', 'wpse239911_only_one_sticky' );

  function wpse239911_only_one_sticky( $post_id ) {
remove_action( 'post_stuck', 'wpse239911_only_one_sticky' );
// prevent loop
delete_option( 'sticky_posts' );
stick_post( $post_id );
 }

  ?>

Answer 1

It sounds like there is a fatal error occurring, but you aren't displaying errors on the page.

To enable debugging you should add the following code to your wp-config.php file.

define( 'WP_DEBUG', true );

*note: if your wp-config.php file already contains an entry for the above, make sure it's set to true.

Once you know what is causing the error you can go about trying to fix it.