disk_free_space
    (PHP 4 >= 4.1.0, PHP 5)
disk_free_space -- Returns available space in directory
Description
float 
disk_free_space ( string directory )
     Given a string containing a directory, this function will return
     the number of bytes available on the corresponding filesystem or
     disk partition.
    
     
Example 1. disk_free_space() example 
<?php // $df contains the number of bytes available on "/" $df = disk_free_space("/");
  // On Windows: disk_free_space("C:"); disk_free_space("D:"); ?>
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    Note: This function will not work on 
remote files as the file to
be examined must be accessible via the servers filesystem.
     See also disk_total_space()