What potential issues can arise when using PHP4 commands in a file with a .php3 extension?

When using PHP4 commands in a file with a .php3 extension, potential issues can arise due to compatibility problems between the older PHP version and the file extension. To solve this issue, it is recommended to update the PHP version to a more recent one that supports the .php extension, which is the standard for PHP files.

// Update the PHP version to a more recent one that supports the .php extension