The behaviour of these functions is affected by settings in php.ini.
| Name | Default | Changeable | Changelog | 
|---|---|---|---|
| xhprof.output_dir | "" | PHP_INI_ALL | 
Here's a short explanation of the configuration directives.
xhprof.output_dir
     string
    Directory used by the default implementation of the iXHProfRuns interface (namely, the XHProfRuns_Default class) for storing XHProf runs.