This error may occour after updating your system and or php.

The logs will show something like this.

PHP Fatal error:  Cannot redeclare hex2bin() in /usr/local/www/squirrelmail/plugins/mail_fetch/functions.php on line xxx

To fix this simply find all instances of hex2bin and replace them with convertHex2bin.

Example using sed.

sed -i '' "s/hex2bin/convertHex2bin/g" /usr/local/www/squirrelmail/plugins/mail_fetch/functions.php