What do the logs on the php server say? Do you see connection attempts?
One possible issue is that you will receive a complete http response from the server not just the message body that you are attempting to decode so there will be lots of spurious characters in the way. Maybe if you include "Serial.print(c)" inside that section you will see whats being sent
For the same reason you should also read the complete response using a while loop.
while(client.available()) {
c = client.read();
serial.print(c); // for debugging
// decode response
}