基本上, 這個修改與之前改給 SMF 用的是同一個, 只是針對 phpBB3 來改.

diff -Nur phpBB3.orig//includes/captcha/plugins/captcha_abstract.php phpBB3/includes/captcha/plugins/captcha_abstract.php
--- phpBB3.orig//includes/captcha/plugins/captcha_abstract.php  2012-01-01 23:00:17.000000000 +0800
+++ phpBB3/includes/captcha/plugins/captcha_abstract.php  2012-01-18 12:17:38.838436361 +0800
@@ -326,7 +326,10 @@
 
   function check_code()
   {
-    return (strcasecmp($this->code, $this->confirm_code) === 0);
+    //return (strcasecmp($this->code, $this->confirm_code) === 0);
+    // we only need first 4 letters, if we got more, reject it
+    if (strlen($this->confirm_code) != 4) return false;
+    return (strcasecmp(substr($this->code, 0, 4), $this->confirm_code) === 0);
   }
 
   function delete_code()
diff -Nur phpBB3.orig//includes/constants.php phpBB3/includes/constants.php
--- phpBB3.orig//includes/constants.php  2012-01-01 23:00:17.000000000 +0800
+++ phpBB3/includes/constants.php  2012-01-18 12:20:43.563129711 +0800
@@ -205,7 +205,7 @@
 @define('CHMOD_EXECUTE', 1);
 
 // Captcha code length
-define('CAPTCHA_MIN_CHARS', 4);
+define('CAPTCHA_MIN_CHARS', 6);
 define('CAPTCHA_MAX_CHARS', 7);
 
 // Additional constants
diff -Nur phpBB3.orig//language/en/common.php phpBB3/language/en/common.php
--- phpBB3.orig//language/en/common.php  2012-01-18 12:10:36.848677118 +0800
+++ phpBB3/language/en/common.php  2012-01-01 23:00:17.000000000 +0800
@@ -123,8 +123,8 @@
   'COLOUR_SWATCH'      => 'Colour swatch',
   'COMMA_SEPARATOR'    => ', ',  // Used in pagination of ACP & prosilver, use localised comma if appropriate, eg: Ideographic or Arabic
   'CONFIRM'        => 'Confirm',
-  'CONFIRM_CODE'      => 'Confirmation code (first 4 letters)',
-  'CONFIRM_CODE_EXPLAIN'  => 'Enter the code exactly as it appears (first 4 letters). All letters are case insensitive.',
+  'CONFIRM_CODE'      => 'Confirmation code',
+  'CONFIRM_CODE_EXPLAIN'  => 'Enter the code exactly as it appears. All letters are case insensitive.',
   'CONFIRM_CODE_WRONG'  => 'The confirmation code you entered was incorrect.',
   'CONFIRM_OPERATION'    => 'Are you sure you wish to carry out this operation?',
   'CONGRATULATIONS'    => 'Congratulations to',
diff -Nur phpBB3.orig//language/zh_cmn_hant/common.php phpBB3/language/zh_cmn_hant/common.php
--- phpBB3.orig//language/zh_cmn_hant/common.php  2012-01-18 12:10:01.745087469 +0800
+++ phpBB3/language/zh_cmn_hant/common.php  2012-01-18 12:08:56.437944350 +0800
@@ -123,8 +123,8 @@
   'COLOUR_SWATCH'  => '顏色取樣',
   'COMMA_SEPARATOR'  => '，',
   'CONFIRM'  => '確認',
-  'CONFIRM_CODE'  => '確認代碼',
-  'CONFIRM_CODE_EXPLAIN'   => '請正確輸入它所顯示的代碼，不必區分大小寫。',
+  'CONFIRM_CODE'  => '確認代碼(前四碼)',
+  'CONFIRM_CODE_EXPLAIN'   => '請正確輸入它所顯示的代碼(前四碼)，不必區分大小寫。',
   'CONFIRM_CODE_WRONG'  => '您輸入的確認代碼有誤。',
   'CONFIRM_OPERATION'  => '您確認要執行這個操作嗎？',
   'CONGRATULATIONS'  => '恭喜您',

修改後, 建議把產生的圖檔改成 "簡單" 的那一個, 因為反正 bot 都看的懂, 沒必要弄那麼複雜, 反而讓人很難看懂.