Squid proxy doesn't launch exteal authentication program

I have written a simple authentication script in PHP, which, as per documentation, takes input from stdin and outputs OK or ERR to stdout.

The problem is, Squid doesn't seem to launch the script at all. It does ask the user to authenticate when connecting (i.e. a login window pops up), but the script doesn't launch (I have it write a line to a log on launch, which doesn't happen)

This is my Squid config (I don't want to include the entire path to the authentication script, but it is a valid one - launching /usr/bin/php [path to]/authenticate.php from the terminal works):

acl SSL_ports port 443
acl Safe_ports port 80 # http
acl Safe_ports port 21 # ftp
acl Safe_ports port 443 # https
acl Safe_ports port 70 # gopher
acl Safe_ports port 210 # wais
acl Safe_ports port 1025-65535 # unregistered ports
acl Safe_ports port 280 # http-mgmt
acl Safe_ports port 488 # gss-http
acl Safe_ports port 591 # filemaker
acl Safe_ports port 777 # multiling http

auth_param basic program /usr/bin/php [path to]/authenticate.php
auth_param basic children 10
auth_param basic realm CZ-Proxy

acl CONNECT method CONNECT
acl auth proxy_auth REQUIRED
acl all_urls url_regex .*

http_access deny !auth
http_access deny !Safe_ports
http_access deny CONNECT !SSL_ports
http_access allow localhost manager
http_access deny manager
http_access allow localhost
http_access allow all

http_port 1234

cache_dir ufs /var/spool/squid 100 16 256
coredump_dir /var/spool/squid

refresh_patte ^ftp:           1440    20%     10080
refresh_patte ^gopher:        1440    0%      1440
refresh_patte -i (/cgi-bin/|?) 0     0%      0
refresh_patte (Release|Packages(.gz)*)$      0       20%     2880
refresh_patte .               0       20%     4320

cache_effective_user proxy
cache_effective_group proxy

Any ideas?

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