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.
这些函数作为 PHP 核心的一部分，无需被安装即可使用。
这些函数的行为受 php.ini 的影响。
表格 1. Mail configuration options
|smtp_port||"25"||PHP_INI_ALL||Available since PHP 4.3.0.|
Used under Windows only: DNS name or IP address of the SMTP server PHP should use for mail sent with the mail() function.
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.
Which "From:" mail address should be used in mail sent from PHP under Windows.
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.
This directive works also under Windows. If set, smtp, smtp_port and sendmail_from are ignored and the specified command is executed.