This error typically occurs when the PHP SimpleXML extension is not properly enabled in PHP, even if it's installed on the server. Here's how to resolve this issue:

1. First, verify SimpleXML installation:
   php -m | grep xml

2. Check PHP configuration:
   php -i | grep simplexml

3. Enable the extension by editing your php.ini file:

   • Locate your php.ini file:
     php --ini

   • Add or uncomment this line in your php.ini:
     extension=php_simplexml.dll ; For Windows

     extension=simplexml.so ; For Linux

4. After making changes:

   • Restart your web server:

     # For Apache on Windows

     net stop apache2.4

     net start apache2.4

     # For IIS

     iisreset

If the extension is already enabled but still not working, try these troubleshooting steps:

1. Check PHP version compatibility

2. Verify file permissions on the SimpleXML module

3. Clear PHP's configuration cache

You can also verify the extension is loaded properly in a PHP file:

<?php
if (extension_loaded('simplexml')) {
    echo "SimpleXML is loaded!";
} else {
    echo "SimpleXML is NOT loaded!";
}

If you're using WHM/cPanel, you can also enable SimpleXML through:

1. WHM → PHP Extensions

2. Find SimpleXML in the list

3. Enable it

4. Restart PHP