• php_value name value
  设置指定的值. 只适合于 PHP_INI_ALL 和 PHP_INI_PERDIR 类型指令. 清除之前设置的值使用 none 标记. Note: 不要使用 php_value 设定布尔值和php_flag (见下文) .
• php_flag name on|off
  用于设置一个布尔配置指令, 只适合于 PHP_INI_ALL 和 PHP_INI_PERDIR 类型指令(和上面的不同就是 php_value 用于设置值(如字符串),而这只能设置 on off)
• php_admin_value name value
  功能和php_value一样,但这个不能用在 .htaccess 文件里, 只能在conf(包括virtualhost)文件里引用
• php_admin_flag name on|off
  同上

有道翻译者,欢迎指正.

以下英文原文

• php_value name value
  Sets the value of the specified directive. Can be used only with PHP_INI_ALL and PHP_INI_PERDIR type directives. To clear a previously set value use none as the value. Note: Don’t use php_value to set boolean values. php_flag (see below) should be used instead.
• php_flag name on|off
  Used to set a boolean configuration directive. Can be used only with PHP_INI_ALL and PHP_INI_PERDIR type directives.
• php_admin_value name value
  Sets the value of the specified directive. This can not be used in .htaccess files. Any directive type set with php_admin_value can not be overridden by .htaccess or virtualhost directives. To clear a previously set value use none as the value.
• php_admin_flag name on|off
  Used to set a boolean configuration directive. This can not be used in .htaccess files. Any directive type set with php_admin_flag can not be overridden by .htaccess or virtualhost directives.

http://www.php.net/manual/en/configuration.changes.php