Question

According to the gmmktime() description in the PHP manual, it uses mktime() internally. Yet when I run the following code, the mktime loop takes just under 9 seconds to run while the gmmktime look takes just under 2 seconds. How can this be?

<?php
$count = 1000000;

$startTime = microtime(true);
for ($i = 0; $i < $count; $i++)
{
  mktime();
}
$endTime = microtime(true);
printf("mktime: %.4f seconds\n", $endTime - $startTime);


$startTime = microtime(true);
for ($i = 0; $i < $count; $i++)
{
  gmmktime();
}
$endTime = microtime(true);
printf("gmmktime: %.4f seconds\n", $endTime - $startTime);

Output:

mktime: 8.6714 seconds
gmmktime: 1.6906 seconds

Answer 1

Most likely, the documentation is lying to you about how gmmktime() is implemented – or it means the C function mktime() is being used.

If we look at the actual code, both gmmktime() and mktime() pass through to an internal php_mktime function, which takes a gmt parameter (set to 1 for gmmktime()). If gmt is zero, then it has to do some extra work (//-comments I have added, others from original code):

/* Initialize structure with current time */
now = timelib_time_ctor();
if (gmt) {
    timelib_unixtime2gmt(now, (timelib_sll) time(NULL));
} else {
    tzi = get_timezone_info(TSRMLS_C);
    now->tz_info = tzi;
    now->zone_type = TIMELIB_ZONETYPE_ID;
    timelib_unixtime2local(now, (timelib_sll) time(NULL));
}

// ... snip shared code

/* Update the timestamp */
if (gmt) {
    // NOTE: Setting the tzi parameter to NULL skips a lot of work in timelib_update_ts
    // (and do_adjust_timezone)
    timelib_update_ts(now, NULL);
} else {
    timelib_update_ts(now, tzi);
}

/* Support for the deprecated is_dst parameter */
if (dst != -1) {
    php_error_docref(NULL TSRMLS_CC, E_DEPRECATED, "The is_dst parameter is deprecated");
    if (gmt) {
        /* GMT never uses DST */
        if (dst == 1) {
            adjust_seconds = -3600;
        }
    } else {
        /* Figure out is_dst for current TS */
        timelib_time_offset *tmp_offset;
        tmp_offset = timelib_get_time_zone_info(now->sse, tzi);
        if (dst == 1 && tmp_offset->is_dst == 0) {
            adjust_seconds = -3600;
        }
        if (dst == 0 && tmp_offset->is_dst == 1) {
            adjust_seconds = +3600;
        }
        timelib_time_offset_dtor(tmp_offset);
    }
}

I suspect what you may find is that, every time you do mktime(), it reopens the timezone description file to read it and get the proper timezone/DST offsets. By using gmmktime(), it skips that by using an internal null timezone for GMT – thus, much faster.