With better words getlastmod() returning the last time the script in which it is being called was modified, it does not require or use a parameter.

Return latest mod time of all included files:

<?php
function get_page_mod_time() {
    $incls = get_included_files();
    $incls = array_filter($incls, "is_file");
    $mod_times = array_map('filemtime', $incls);
    $mod_time = max($mod_times);

    return $mod_time;
}
?>

If you use register_shutdown_function() on certain SAPIs, various filesystem-related things inside the shutdown function might do unexpected things, one of which being this function can return false.

On the other hand getlastmod() apparently caches the return value, so if you use it at least once in normal code it should work for the remainder of the request.

DO NOT use this function unless you are absolutely sure both your Apache and PHP have been compiled with the same value for -DFILE_OFFSET_BITS.

If not, this function will return the access time (or maybe even garbage) instead of the modification time due do Apache and PHP using different versions of the stat structure.

This is true regardless of Apache and PHP version.

To be on the safe side, always use the workaround already posted below:
filemtime($_SERVER['SCRIPT_FILENAME'])

Setting the 'Last-Modified' header:
<?php
setlocale(LC_TIME, "C");
$ft = filemtime ('referencefile');
$localt = mktime ();
$gmtt = gmmktime ();
$ft = $ft - $gmtt + $localt;
$modified = strftime ("%a, %d %b %Y %T GMT", $ft);
?>