In that case, assuming you are storing the entries on per line, with the most recent entries appended to the end of the file, you could simply read the file into an array, and loop through it backwards:
<?php
$entries = file( '/path/to/file.dat' );
Reads file.dat into the array $entries (1 line per element)
for( $loop = count( $entries ); $loop >= 0; $loop-- )
{
print $entries[$loop] . "\n";
Prints the entire line
}
?>
Hope this helps!
-Rich