PHP exec ping doesn't properly work on Unix server - php

Question

I want to ping a range of ips on the server that runs Unix
exec('ping -c 4 '.$ip, $output, $return_var);
Problem is that, it doesn't properly ping the ips that actually gets pinged but can't be found on ns_lookup in cmd. what I mean is that some of these ips gets pinged and some not. On localhost, that runs windows, it works fine.
this is the $output array I get when I ping that ip on server (runs Unix):
array(5) {
[0]=>
string(54) "PING 91.208.144.2 (91.208.144.2) 56(84) bytes of data."
[1]=>
string(0) ""
[2]=>
string(36) "--- 91.208.144.2 ping statistics ---"
[3]=>
string(64) "4 packets transmitted, 0 received, 100% packet loss, time 2999ms"
[4]=>
string(0) ""
}
that's the $output array I get when I ping it on localhost (on Windows):
array(11) {
[0]=>
string(0) ""
[1]=>
string(43) "Pinging 91.208.144.2 with 32 bytes of data:"
[2]=>
string(50) "Reply from 91.208.144.2: bytes=32 time=9ms TTL=248"
[3]=>
string(51) "Reply from 91.208.144.2: bytes=32 time=10ms TTL=248"
[4]=>
string(51) "Reply from 91.208.144.2: bytes=32 time=11ms TTL=248"
[5]=>
string(50) "Reply from 91.208.144.2: bytes=32 time=8ms TTL=248"
[6]=>
string(0) ""
[7]=>
string(33) "Ping statistics for 91.208.144.2:"
[8]=>
string(56) " Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),"
[9]=>
string(46) "Approximate round trip times in milli-seconds:"
[10]=>
string(48) " Minimum = 8ms, Maximum = 11ms, Average = 9ms"
}
exec and ping is available on server cuz it works perfect with the ips that gets pinged and can be looked up in cmd.
Any ideas ?