Why does the usage of namespaces in a PHP class lead to issues in code that doesn't incorporate them?

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The usage of namespaces in a PHP class leads to issues in code that doesn't incorporate them because it changes the way classes are referred to and resolved. If a class is defined within a namespace, it is only accessible within that namespace unless it is explicitly imported or referenced with the fully qualified namespace. This can lead to naming conflicts and unresolved class references in code that doesn't use namespaces, as the namespace is not recognized or considered when resolving class names. Therefore, it is important to use namespaces consistently throughout a project to avoid conflicts and ensure proper class resolution.