This will trigger a warning and return false if you pass it a non-symlink. If the file doesn't exist, it will trigger a differently worded warning.
mslade@jupiter ~$ touch a
mslade@jupiter ~$ ln -s a b
mslade@jupiter ~$ ls -l {a,b}
-rw------- 1 mslade mslade 0 2009-06-10 15:27 a
lrwxrwxrwx 1 mslade mslade 1 2009-06-10 15:27 b -> a
mslade@jupiter ~$ php -r "var_dump(readlink('b'));"
string(1) "a"
mslade@jupiter ~$ php -r "var_dump(readlink('a'));"
Warning: readlink(): Invalid argument in Command line code on line 1
bool(false)
mslade@jupiter ~$ php -r "var_dump(readlink('c'));"
Warning: readlink(): No such file or directory in Command line code on line 1
bool(false)
