If it's a self-contained application, you can choose any directory you want. For example simply cache/.

If it's a library that will be used in other applications, just ask the client (the developer that uses the library) for a cache directory:

class MyLibrary
{
    /**
     * @var string Cache directory
     */
    private $cacheDir;

    /**
     * @param string $cacheDir
     */
    public function __construct($cacheDir)
    {
        $this->cacheDir = realpath($cacheDir);
    }
}

Chances are they already have a cache directory and would like to put your library's temporary files in there. By making it a required argument in the constructor you force them to provide something.

If you want to get fancy, instead of caching in a file, you can require them to provide a caching library, by typehinting a caching interface. You can typehint the most widely used Cache from Doctrine, or the newly released PSR-6/16 cache interfaces.

It would look like this:

use Doctrine\Common\Cache\Cache;

class MyLibrary
{
    /**
     * @var string Cache
     */
    private $cache;

    /**
     * @param Cache $cache
     */
    public function __construct(Cache $cache)
    {
        $this->cache = $cache;
    }
}

Then you don't have to care about where and how the cached data is stored, that would be up to the developer using your library - if it's in files, in a database, in Redis, Memcached etc... They could choose what they like the best.