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