readyState= 1
this.status= 0
readyState= 2
this.status= 200
readyState= 3
this.status= 200
readyState= 4
this.status= 200

These are the responses I get, and in the end it finally responds with 4 and 200. My php function just echoes "hello from php". I used ob_clean() before echo and wp_die() as per Codex recommendations because it said there may be unnecessary responses mixed in with ajax but still didn't help at all.