touch

Note that when PHP is called by f.e. apache or nginx instead of directly from the command line, touch() will not prefix the location of the invoking script, so the supplied filename must contain an absolute path.
With script started from /home/user/www, this will not touch "/home/user/www/somefile":
<?php
 touch( 'somefile' );
?>

But this will:
<?php
 touch( __DIR__ . '/somefile' );
?>

Update the access time without updating the modified time:
Unix command: touch -a filename
PHP: touch(filename, date('U', filemtime(filename)), time())

I've been trying to set a filemtime into the future with touch() on PHP5.
It seems touch $time has a future limit around 1000000 seconds (11 days or so). Beyond this point it reverts to a previous $time.
It doesn't make much sense but I could save you hours of time.
$time = time()+1500000;
touch($cachedfile,$time);

At least on Linux, touch will not change the time on a symlink itself, but on the file/directory it points to. The only way to work around this is to unlink the symlink, then recreate it.
It took a bit of searching to discover this. The OS itself provides no way to do it. Many people wondered why anyone would want to do this. I use symlinks inside a web tree to point to files outside the web tree. After a certain length of time has passed, I want the symlinks to die, so the files cannot be successfully hotlinked.

I needed to use this to touch the /etc/cron.d directory when I updated some files in there. I know the docs say this isn't necessary, but I'm finding that i need to do it in order form my changes to be picked up quickly. 
I ran into the permissions error as well and I found that using chmod 777 /etc/cron.d does the trick.
So, you should be able to use the PHP touch function on a directory that has open write access.
Of course, this isn't the most secure approach, but in our application it's not a big deal for that folder to not be super secure.

To touch a file without being owner, it is much easier:
<?php
function touchFile($file) {
 fclose(fopen($file, 'a'));
}
?>

Actually, Glen is right, PHP won't touch if it is not the current owner of the file, even if the directory and files are writeable by the PHP user.

A better explanation:
For file $file and UNIX time stored in vars $access and $modified
- change only access time
\touch($file, \filemtime($file), $access);
- change only modified time
\touch($file, $modified, \fileatime($file));
- change both access and modified time
\touch($file, $modified, $access);
Seeing the results:
//use a session cookie stored in a custom folder
$file = '/var/www/test_com/session/sess_qfn587cudfpgsijm1bs4d81s75';
echo 'stats for sess_qfn587cudfpgsijm1bs4d81s75<br/>';
\clearstatcache();
echo 'access: '.\date("Y-m-d H:i:s", \fileatime($file)).'<br/>';
echo 'modified: '.\date("Y-m-d H:i:s", \filemtime($file)).'<br/>';
echo 'change access to now, modified +1 hour<br/>';
\touch($x, \filemtime($file)+3600, time());
\clearstatcache();
echo 'access: '.\date("Y-m-d H:i:s", \fileatime($file)).'<br/>';
echo 'modified: '.\date("Y-m-d H:i:s", \filemtime($file)).'<br/>';
Notice the double call to clearstatcache()!

I have found out that setting a negative *mtime* deletes the file. The following code always deletes the file at $path while $touch returns true.
<?php
$path = '/folder/file';
$timestamp = -1;
$touch = touch($path, $timestamp);
?>

Running PHP 7.4.5

Only way to change modification date in catalogue is to create file in via touch() and dalete it with unlink():
<?php
$dir = 'temp';
$files1 = scandir($dir);
$files1 = array_slice($files1, 2);
foreach ($files1 as $key => $val) 
{
 if (!is_dir($val)) continue;
 if (!touch($val)) 
 {
 touch($val . "/plik.txt");
 unlink($val . "/plik.txt");
 }
}
?>

Neat little script that will give you a list of all modified files in a certain folder after a certain date:
$filelist = Array();
$filelist = list_dir("d:\\my_folder");
for($i=0;$i<count($filelist);$i++){
 $test = Array();
 $test = explode("/",date("m/d/Y",filemtime($filelist[$i])));
//example of files that are later then
//06/17/2002
 if(($test[2] > 2001) && ($test[1] > 16) && ($test[0] > 5)){
 echo $filelist[$i]."\r\n";
 }
 clearstatcache();
}
function list_dir($dn){
 if($dn[strlen($dn)-1] != '\\') $dn.='\\';
 static $ra = array();
 $handle = opendir($dn);
 while($fn = readdir($handle)){
 if($fn == '.' || $fn == '..') continue;
 if(is_dir($dn.$fn)) list_dir($dn.$fn.'\\');
 else $ra[] = $dn.$fn;
 }
 closedir($handle);
 return $ra;
}

In unix on the command-line, you can touch files you don't own - but like other comments on this page state - PHP's built in touch won't work.
I simple alternative (on unix):
<?php
 function touch_it_good($filename)
 {
 exec("touch {$filename}");
 }
?>

Note: the script to touch a file you don't own will change it's owner so ensure permissions are correct or you could lose access to it

Important info:
touch() used on a directory always returns FALSE and prints "Permission denied" on NTFS and FAT Filesystem (tested on winXP).