When using PHP as an Apache module, you can also change the configuration settings using directives in Apache configuration files (e.g. httpd.conf) and .htaccess files. You will need "AllowOverride Options" or "AllowOverride All" privileges to do so.
    There are several Apache directives that allow you
    to change the PHP configuration from within the Apache configuration
    files.  For a listing of which directives are
    PHP_INI_ALL, PHP_INI_PERDIR,
    or PHP_INI_SYSTEM, have a look at the
    List of php.ini directives appendix.
   
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_valueto 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 ini_set().
        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 ini_set().
       
Example #1 Apache configuration example
<IfModule mod_php5.c> php_value include_path ".:/usr/local/lib/php" php_admin_flag engine on </IfModule> <IfModule mod_php4.c> php_value include_path ".:/usr/local/lib/php" php_admin_flag engine on </IfModule>
     PHP constants do not exist outside of PHP. For example, in
     httpd.conf you can not use PHP constants
     such as E_ALL or E_NOTICE
     to set the error_reporting
     directive as they will have no meaning and will evaluate to
     0. Use the associated bitmask values instead.
     These constants can be used in php.ini
    
    When running PHP on Windows, the configuration values can be
    modified on a per-directory basis using the Windows registry. The
    configuration values are stored in the registry key
    HKLM\SOFTWARE\PHP\Per Directory Values,
    in the sub-keys corresponding to the path names. For example, configuration
    values for the directory c:\inetpub\wwwroot would
    be stored in the key HKLM\SOFTWARE\PHP\Per Directory
    Values\c\inetpub\wwwroot. The settings for the
    directory would be active for any script running from this
    directory or any subdirectory of it. The values under the key
    should have the name of the PHP configuration directive and the
    string value. PHP constants in the values are not parsed.
    However, only configuration values changeable in
    PHP_INI_USER can be set
    this way, PHP_INI_PERDIR values can not.
   
Regardless of how you run PHP, you can change certain values at runtime of your scripts through ini_set(). See the documentation on the ini_set() page for more information.
If you are interested in a complete list of configuration settings on your system with their current values, you can execute the phpinfo() function, and review the resulting page. You can also access the values of individual configuration directives at runtime using ini_get() or get_cfg_var().