I recently made an interesting observation:
It seems that `session_start()` can return `true` even if the session was not properly created. In my case, the disk storage was full and so the session data could not be written to disk. I had some logic that resulted in an infinite loop when the session was not written to disk.
To check if the session really was saved to disk I used:
```
<?php
function safe_session_start() {
 # Attempt to start a session
 if (!@\session_start()) return false;
 #
 # Check if we need to perform
 # the write test.
 #
 if (!isset($_SESSION['__validated'])) {
 $_SESSION['__validated'] = 1;
 # Attempt to write session to disk
 @\session_write_close();
 # Unset the variable from memory.
 # This step may be unnecessary
 unset($_SESSION['__validated']);
 # Re-start session
 @\session_start();
 # Check if variable value is retained
 if (!isset($_SESSION['__validated'])) {
 # Session was not written to disk
 return false;
 }
 }
 return true;
}
if (!safe_session_start()) {
 # Sessions are probably not written to disk...
 # Handle error accordingly.
}
?>
```
Took me quite a while to figure this out. 
Maybe it helps someone!