Add error handling for missing or unreadable WordPress files in core commands

features/core-install.feature

@@ -322,3 +322,69 @@ Feature: Install WordPress core
     """
     Success: Switched to 'Rockfield' theme.
     """
+
+  Scenario: Core install should provide helpful error when upgrade.php is missing
+    Given an empty directory
+    And WP files
+    And wp-config.php
+    And a database
+
+    When I run `rm wp-admin/includes/upgrade.php`
+    Then the return code should be 0
+
+    When I try `wp core install --url=example.org --title=Test --admin_user=testadmin --admin_email=testadmin@example.com --admin_password=testpass`
+    Then STDERR should contain:
+      """
+      Error: WordPress installation is incomplete. The file
+      """
+    And STDERR should contain:
+      """
+      wp-admin/includes/upgrade.php' is missing.
+      """
+    And the return code should be 1
+
+  Scenario: Core install should provide helpful error when upgrade.php is not readable
+    Given an empty directory
+    And WP files
+    And wp-config.php
+    And a database
+
+    When I run `chmod 000 wp-admin/includes/upgrade.php`
+    Then the return code should be 0
+
+    When I try `wp core install --url=example.org --title=Test --admin_user=testadmin --admin_email=testadmin@example.com --admin_password=testpass`
+    Then STDERR should contain:
+      """
+      Error: Cannot read WordPress installation file
+      """
+    And STDERR should contain:
+      """
+      wp-admin/includes/upgrade.php'. Check file permissions.
+      """
+    And the return code should be 1
+
+    When I run `chmod 644 wp-admin/includes/upgrade.php`
+    Then the return code should be 0
+
+  Scenario: Core install should provide helpful error when WordPress file has fatal error from missing extension
+    Given an empty directory
+    And WP files
+    And wp-config.php
+    And a database
+
+    # Simulate a missing mysqli extension by replacing upgrade.php with code that
+    # triggers a fatal error mimicking what happens when mysqli_connect is called
+    # but the extension is not available. This reproduces the original bug report scenario
+    When I run `echo "<?php trigger_error('Call to undefined function mysqli_connect()', E_USER_ERROR);" > wp-admin/includes/upgrade.php`
+    Then the return code should be 0
+
+    When I try `wp core install --url=example.org --title=Test --admin_user=testadmin --admin_email=testadmin@example.com --admin_password=testpass`
+    Then STDERR should contain:
+      """
+      Fatal error: Call to undefined function mysqli_connect()
+      """
+    And STDERR should contain:
+      """
+      Error: There has been a critical error on this website
+      """
+    And the return code should be 255

src/Core_Command.php

@@ -684,7 +684,7 @@ function wp_new_blog_notification() {
 			add_filter( 'send_site_admin_email_change_email', '__return_false' );
 		}
 
-		require_once ABSPATH . 'wp-admin/includes/upgrade.php';
+		$this->require_upgrade_file( 'WordPress installation' );
 
 		$defaults = [
 			'title'          => '',
@@ -744,7 +744,7 @@ function wp_new_blog_notification() {
 	private function multisite_convert_( $assoc_args ) {
 		global $wpdb;
 
-		require_once ABSPATH . 'wp-admin/includes/upgrade.php';
+		$this->require_upgrade_file( 'multisite conversion' );
 
 		$domain = self::get_clean_basedomain();
 		if ( 'localhost' === $domain && ! empty( $assoc_args['subdomains'] ) ) {
@@ -1263,7 +1263,7 @@ public function update( $args, $assoc_args ) {
 				|| Utils\get_flag_value( $assoc_args, 'force' )
 				|| ( $update->locale ?: 'en_US' ) !== ( self::get_wp_details()['wp_local_package'] ?: 'en_US' ) ) ) {
 
-			require_once ABSPATH . 'wp-admin/includes/upgrade.php';
+			$this->require_upgrade_file( 'WordPress core update' );
 
 			// Prevent async translation updates which output HTML.
 			add_action(
@@ -1545,7 +1545,7 @@ public function update_db( $args, $assoc_args ) {
 			}
 			WP_CLI::success( "WordPress database upgraded on {$success}/{$total} sites." );
 		} else {
-			require_once ABSPATH . 'wp-admin/includes/upgrade.php';
+			$this->require_upgrade_file( 'WordPress database update' );
 
 			/**
 			 * @var string $wp_current_db_version
@@ -1949,11 +1949,35 @@ function () use ( $new_zip_file ) {
 	}
 
 	/**
-	 * Checks if a WP_Error is related to the core_updater.lock.
+	 * Safely requires the WordPress upgrade.php file with error handling.
+	 *
+	 * This method checks for file existence and readability before requiring,
+	 * and registers a shutdown function to catch fatal errors during file loading
+	 * (e.g., missing PHP extensions or other runtime issues).
 	 *
-	 * @param \WP_Error $error The error object to check.
-	 * @return bool True if the error is related to the lock, false otherwise.
+	 * @param string $context Context for error messages (e.g., 'installation', 'upgrade', 'database update').
 	 */
+	private function require_upgrade_file( $context = 'WordPress operation' ) {
+		$upgrade_file = ABSPATH . 'wp-admin/includes/upgrade.php';
+
+		if ( ! file_exists( $upgrade_file ) ) {
+			WP_CLI::error( "WordPress installation is incomplete. The file '{$upgrade_file}' is missing." );
+		}
+
+		if ( ! is_readable( $upgrade_file ) ) {
+			WP_CLI::error( "Cannot read WordPress installation file '{$upgrade_file}'. Check file permissions." );
+		}
+
+		// phpcs:ignore WordPressVIPMinimum.Files.IncludingFile.UsingVariable -- Path comes from WordPress itself.
+		require_once $upgrade_file;
+	}
+
+	/**
+		* Checks if a WP_Error is related to the core_updater.lock.
+		*
+		* @param \WP_Error $error The error object to check.
+		* @return bool True if the error is related to the lock, false otherwise.
+		*/
 	private static function is_lock_error( $error ) {
 		// Check for the 'locked' error code used by WordPress Core
 		if ( 'locked' === $error->get_error_code() ) {