This function returns the resolved absolute path name for the external entity given by name. If the entity given by name is not defined or is not a file or graphic entity name, the function returns the null string. The doc parameter specifies the identifier of the document tree to query. If omitted or 0, the current document is used. This callback should always be set globally.

For example,

would return the full path and file name of the prodname file entity in the $path variable.