URI for this namespace is http://xml.zope.org/namespaces/metal. To use metal: attribute prefix in XML you're required to declare it:
<html xmlns:metal="http://xml.zope.org/namespaces/metal" …>PHPTAL does not enforce this requirement.
METAL stands for 'Macro Extension for TAL'. This namespace allows template designers to define and call macros. Macros can be used to output data recursively or to include code from other template files.