WP_MEMORY_LIMIT在wp-config中不生效，仅在default-constants.php生效问题

Let's break down how to fix this issue properly—because modifying core files like default-constants.php is never a sustainable solution (as you've noticed, WordPress updates will always overwrite those changes).

Why Your wp-config.php Setting Isn't Overriding the Default #

Normally, any constant you define in wp-config.php should take priority over the defaults in default-constants.php. That core file uses defined() checks to only set the constant if it hasn't already been defined elsewhere. If your setting isn't sticking, here are the most likely culprits:

• A typo in your define() statement (double-check the constant name WP_MEMORY_LIMIT and value format, e.g., '256M' with proper quotes)
• Another plugin or theme is resetting the memory limit after your wp-config.php definition
• You accidentally placed the define statement in the wrong section of wp-config.php

Step-by-Step Fixes #

1. Correct Your wp-config.php Setup (No Core File Edits!)

First, delete any changes you made to default-constants.php—we're going to do this the right way.

Make sure your WP_MEMORY_LIMIT define is placed before the line that says require_once(ABSPATH . 'wp-settings.php'); (usually right before the closing ?> if it exists). Your code should look like this:

define( 'WP_DEBUG', true );
// Add your memory limit define right here
define( 'WP_MEMORY_LIMIT', '256M' );

/* That's all, stop editing! Happy publishing. */

define( 'ABSPATH', dirname( __FILE__ ) . '/' );
require_once( ABSPATH . 'wp-settings.php' );

Double-check for typos—even a missing quote or lowercase letter can break this.

2. Force the Memory Limit via Code (If wp-config.php Still Fails)

If plugins or themes are overriding your setting, you can enforce it using a custom function. Add this to your active theme's functions.php file (or better yet, a must-use plugin so it doesn't get lost if you switch themes):

// Increase memory limit for admin area (adjust value as needed)
function custom_increase_memory_limit() {
    if ( is_admin() ) {
        wp_raise_memory_limit( 'admin' );
        // Or directly set it if wp_raise_memory_limit isn't enough:
        // ini_set( 'memory_limit', '256M' );
    }
}
add_action( 'admin_init', 'custom_increase_memory_limit' );

Must-use plugins live in wp-content/mu-plugins/—create that folder if it doesn't exist, then add a file like increase-memory.php with the code above. This ensures the setting loads before any other plugins or themes.

3. Check for Conflicting Plugins/Themes

Temporarily disable all plugins and switch to a default theme (like Twenty Twenty-Four). If the memory limit starts working, re-enable plugins one by one to find which one is overriding your setting. Some optimization or security plugins might reset memory limits as part of their functionality.

Key Takeaway #

Never edit WordPress core files—they'll be overwritten on every update. All customizations should happen in wp-config.php, your theme's functions.php, or custom/must-use plugins.

内容的提问来源于stack exchange，提问作者Papejoo