is_link

To find out whether a file is hardlinked to another filename, check the number of links of the stat() output. If it is >1 there is another filename for that file.
To find out whether two filenames are pointing to the same file, check the inode number of those 2 filenames. If it is equal, the 2 filenames are hardlinked together.

A workaround for Windows LNK files:
<?php
function _is_link($filename)
{
 if(is_link($filename))
 return true;
 $ext = substr(strrchr($filename, '.'), 1);
 if(strtolower($ext) == 'lnk')
 {
 return (_readlink($filename) ? true : false);
 }
 return false;
}
function _readlink($file)
{
 if(file_exists($file))
 {
 if(is_link($file))
 {
 return readlink($file);
 }
 // Get file content
 $handle = fopen($file, "rb");
 $buffer = array();
 while(!feof($handle))
 {
 $buffer[] = fread($handle, 1);
 }
 fclose($handle);
 // Test magic value and GUID
 if(count($buffer) < 20)
 return false;
 if($buffer[0] != 'L')
 return false;
 if((ord($buffer[4]) != 0x01) ||
 (ord($buffer[5]) != 0x14) ||
 (ord($buffer[6]) != 0x02) ||
 (ord($buffer[7]) != 0x00) ||
 (ord($buffer[8]) != 0x00) ||
 (ord($buffer[9]) != 0x00) ||
 (ord($buffer[10]) != 0x00) ||
 (ord($buffer[11]) != 0x00) ||
 (ord($buffer[12]) != 0xC0) ||
 (ord($buffer[13]) != 0x00) ||
 (ord($buffer[14]) != 0x00) ||
 (ord($buffer[15]) != 0x00) ||
 (ord($buffer[16]) != 0x00) ||
 (ord($buffer[17]) != 0x00) ||
 (ord($buffer[18]) != 0x00) ||
 (ord($buffer[19]) != 0x46))
 {
 return false;
 }
 $i = 20;
 if(count($buffer) < ($i + 4))
 return false;
 $flags = ord($buffer[$i]);
 $flags = $flags | (ord($buffer[++$i]) << 8);
 $flags = $flags | (ord($buffer[++$i]) << 16);
 $flags = $flags | (ord($buffer[++$i]) << 24);
 $hasShellItemIdList = ($flags & 0x00000001) ? true : false;
 $pointsToFileOrDir = ($flags & 0x00000002) ? true : false;
 if(!$pointsToFileOrDir)
 return false;
 if($hasShellItemIdList)
 {
 $i = 76;
 if(count($buffer) < ($i + 2))
 return false;
 $a = ord($buffer[$i]);
 $a = $a | (ord($buffer[++$i]) << 8);
 
 }
 $i = 78 + 4 + $a;
 if(count($buffer) < ($i + 4))
 return false;
 $b = ord($buffer[$i]);
 $b = $b | (ord($buffer[++$i]) << 8);
 $b = $b | (ord($buffer[++$i]) << 16);
 $b = $b | (ord($buffer[++$i]) << 24);
 $i = 78 + $a + $b;
 if(count($buffer) < ($i + 4))
 return false;
 $c = ord($buffer[$i]);
 $c = $c | (ord($buffer[++$i]) << 8);
 $c = $c | (ord($buffer[++$i]) << 16);
 $c = $c | (ord($buffer[++$i]) << 24);
 $i = 78 + $a + $b + $c;
 if(count($buffer) < ($i +1))
 return false;
 $linkedTarget = "";
 for(;$i < count($buffer); ++$i)
 {
 if(!ord($buffer[$i]))
 break;
 $linkedTarget .= $buffer[$i];
 }
 if(empty($linkedTarget))
 return false;
 
 return $linkedTarget;
 }
 return false;
}
if(_is_link('test.lnk'))
{
 echo _readlink('test.lnk');
}
?>

Careful when using is_link() in thread-safe builds with the self-referential directory entry ".". On ZTS builds is_link("/home/symlink/.") will return true whereas it returns false on non-ZTS builds.

On windows, take care that is_link returns false for Junctions.
Ways of telling apart a directory from a junction include doing both a stat() and and lstat() call and checking if there is any difference in the results