What common mistakes should be avoided when defining namespaces and using Composer in PHP projects?

Common mistakes when defining namespaces and using Composer in PHP projects include not following PSR-4 autoloading standards, using incorrect namespace declarations, and not updating the Composer autoloader after making changes to namespaces. To avoid these mistakes, ensure that namespaces follow the PSR-4 standard, correctly declare namespaces in files, and run `composer dump-autoload` after making any changes to namespaces.

// Incorrect namespace declaration
namespace MyProject;

// Correct namespace declaration
namespace MyProject\Subfolder;

// Run this command after making changes to namespaces
composer dump-autoload