The behaviour of these functions is affected by settings in php.ini.
| Name | Default | Changeable | Changelog | 
|---|---|---|---|
| mail.add_x_header | "0" | PHP_INI_PERDIR | |
| mail.log | NULL | PHP_INI_SYSTEM|PHP_INI_PERDIR | |
| mail.force_extra_parameters | NULL | PHP_INI_SYSTEM|PHP_INI_PERDIR | |
| SMTP | "localhost" | PHP_INI_ALL | |
| smtp_port | "25" | PHP_INI_ALL | |
| sendmail_from | NULL | PHP_INI_ALL | |
| sendmail_path | "/usr/sbin/sendmail -t -i" | PHP_INI_SYSTEM | 
Here's a short explanation of the configuration directives.
mail.add_x_header
    bool
   
     Add X-PHP-Originating-Script that will include UID of
     the script followed by the filename.
    
mail.log
    string
   
     The path to a log file that will log all mail() calls.
     Log entries include the full path of the script, line number,
     To address and headers.
    
mail.force_extra_parameters
    string
   Force the addition of the specified parameters to be passed as extra parameters to the sendmail binary. These parameters will always replace the value of the 5th parameter to mail().
SMTP
    string
   Used under Windows only: host 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.
    
sendmail_from
    string
   
     Which "From:" mail address should be used in mail sent
     directly via SMTP (Windows only).
     This directive also sets the "Return-Path:" header.
    
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.
This directive works also under Windows. If set, smtp, smtp_port and sendmail_from are ignored and the specified command is executed.