PHP Fatal Errors-causes and how to debug

A PHP fatal error stops the entire script and displays a "Fatal error" or blank white page. Unlike warnings (non-fatal), fatal errors prevent your page from loading at all.

Common fatal errors

• Call to undefined function: Plugin calls function that doesn't exist
• Allowed memory exceeded: PHP memory limit too low (default 128M)
• Class not found: Missing plugin file or incorrect include path
• Maximum execution time exceeded: Script takes too long
• Syntax error: Parse error in PHP code (missing semicolon, brace)
• Cannot redeclare function: Function defined twice (duplicate plugin code)
• Cannot use empty string as offset: Array access error in code

Enable PHP debug logging

For WordPress:

Add to wp-config.php (before "That's all"):

define('WP_DEBUG', true);
define('WP_DEBUG_LOG', true);
define('WP_DEBUG_DISPLAY', false);  // don't show errors to visitors

Error details will appear in /wp-content/debug.log

For generic PHP:

Add to .htaccess:

php_flag display_errors on
php_value log_errors on
php_value error_log /home/username/public_html/php-error.log

Read error logs

Via cPanel:

1. Go to Logs → Raw Access Logs or Error Log
2. Download the error log file
3. Search for "Fatal" or "Call to undefined"

Via command line (VPS):

tail -50 /var/log/php-errors.log
tail -50 /home/username/public_html/debug.log

How to fix fatal errors

Undefined function

• Plugin missing required dependency
• Fix: Install the required plugin or disable the dependent plugin

Memory limit exceeded

• Increase in wp-config.php: define('WP_MEMORY_LIMIT', '256M');
• Or increase in .htaccess: php_value memory_limit 256M

Class not found

• Plugin file didn't load
• Fix: Check plugin is activated and main file is spelled correctly

Syntax error (parse error)

• Error log will show exact line number
• Check that line for missing semicolon, brace, quote

WordPress-specific fatal errors

White screen of death (WSOD) = fatal error

Enable WP_DEBUG to see what caused it. Usually a plugin conflict.

Disable all plugins to test:

1. Via FTP: Rename /wp-content/plugins to /wp-content/plugins-old
2. Visit your site - does it work?
3. If yes, a plugin caused it. Rename back and disable plugins one by one

Check PHP version:

Some old plugins don't support PHP 7.4+. Upgrade the plugin or downgrade PHP version (via cPanel).

Related: Memory limit errors | Parse errors | WordPress troubleshooting

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