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Fatal error: Uncaught Error: Call to undefined function wp_get_environment_type()

The error occurs when WordPress tries to call wp_get_environment_type but the function is not available. This happens because the core files that define the function are missing, outdated, or not loaded when the call is made. The result is a fatal PHP error that stops execution.

The function was added in WordPress 5.5. Installations running older versions, corrupted core, or plugins that invoke the function before WordPress finishes loading will trigger the error.

Symptoms

[dashicons-warning]
White screen of death
The site shows a blank page or a 500 error response.
[dashicons-editor-code]
Fatal error message
Browser displays "Fatal error: Uncaught Error: Call to undefined function wp_get_environment_type()".
[dashicons-admin-generic]
Admin area inaccessible
Any wp-admin URL returns the fatal error.

Common Causes

WordPress version older than 5.5
Core does not contain the wp_get_environment_type function.
Corrupted core files
The file that defines the function is missing or damaged.
Plugin loads before core
A plugin such as Yoast SEO calls the function too early in the load process.
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