需求
     For the Mail functions to be available, PHP must have access to the
     sendmail binary on your system during compile time.
     If you use another mail program, such as qmail or postfix, be sure to
     use the appropriate sendmail wrappers that come with them.  PHP will
     first look for sendmail in your PATH, and then in
     the following:  
     /usr/bin:/usr/sbin:/usr/etc:/etc:/usr/ucblib:/usr/lib.
     It's highly recommended to have sendmail available from your
     PATH.  Also, the user that compiled PHP must have
     permission to access the sendmail binary.
    
运行时配置
这些函数的行为受到全局配置文件 php.ini 的影响。
 
表格 1. Mail configuration options
| Name | Default | Changeable | 
|---|
| SMTP | "localhost" | PHP_INI_ALL | 
| smtp_port | "25" | PHP_INI_ALL | 
| sendmail_from | NULL | PHP_INI_ALL | 
| sendmail_path | DEFAULT_SENDMAIL_PATH | PHP_INI_SYSTEM | 
 For further details and definition of the PHP_INI_* constants see
 
ini_set().
 
以下是该配置选项的简要解释。
 
- SMTP
    string
- 
     Used under Windows only: DNS name or IP address of the SMTP server PHP
     should use for mail sent with the mail() function.
     
- smtp_port
    int
- 
     Used under Windows only: Number of the port to connect to the server
     specified with the SMTP setting when sending mail
     with mail(); defaults to 25. Only available since
     PHP 4.3.0.
     
- sendmail_from
    string
- 
     Which "From:" mail address should be used in mail sent from
     PHP under Windows.
     
- sendmail_path
    string
- 
     Where the sendmail program can be found,
     usually /usr/sbin/sendmail or
     /usr/lib/sendmail.
     configure does an honest attempt of
     locating this one for you and set a default, but if it fails,
     you can set it here.
     - 
     Systems not using sendmail should set this directive to the
     sendmail wrapper/replacement their mail system offers, if any.
     For example, Qmail
     users can normally set it to
     /var/qmail/bin/sendmail or 
     /var/qmail/bin/qmail-inject.
     - 
     qmail-inject does not require any option to
     process mail correctly.