Are there restrictions on PHP functions I can use in PHP customizations?

Environment:

All PHP/CPHP customizations, Process Designer, Custom Process Model (CPM)
May 2012 version and newer

Resolution:

Several standard PHP functions are excluded from use in customizations using PHP/CPHP.

dl()
system()
symlink()
shell_exec()
proc_open()
passthru()
link()
lchown()
lchgrp()
ini_set()
exec()
chown()
chmod()
chown()
filter_var()
fsockopen()
getmypid()
mb_substr()
hash_hmac()
simplexml_load_string()
readfile()

For further details see

Answer ID 10517: PHP fsockopen and pfsockopen restricted in 18D release

Answer ID 9825: Cannot use the hash_hmac function in PHP