Here is a quick parse_ini_file wrapper to add extend support to save typing and redundancy.
<?php
    /**
     * Parses INI file adding extends functionality via ":base" postfix on namespace.
     *
     * @param string $filename
     * @return array
     */
    function parse_ini_file_extended($filename) {
        $p_ini = parse_ini_file($filename, true);
        $config = array();
        foreach($p_ini as $namespace => $properties){
            list($name, $extends) = explode(':', $namespace);
            $name = trim($name);
            $extends = trim($extends);
            // create namespace if necessary
            if(!isset($config[$name])) $config[$name] = array();
            // inherit base namespace
            if(isset($p_ini[$extends])){
                foreach($p_ini[$extends] as $prop => $val)
                    $config[$name][$prop] = $val;
            }
            // overwrite / set current namespace values
            foreach($properties as $prop => $val)
            $config[$name][$prop] = $val;
        }
        return $config;
    }
?>

Treats this ini:
<?php 
/*
[base]
host=localhost
user=testuser
pass=testpass
database=default

[users:base]
database=users

[archive : base]
database=archive
*/
?>
As if it were like this:
<?php
/*
[base]
host=localhost
user=testuser
pass=testpass
database=default

[users:base]
host=localhost
user=testuser
pass=testpass
database=users

[archive : base]
host=localhost
user=testuser
pass=testpass
database=archive
*/
?>

To make the content available in every corner of you application I use a user defined constant. $SETTINGS. It is initialized like this
<?php

define( 'SETTINGS", parse_ini_file('settings.ini', true) );

?>
With the proper settings.ini file you can now do stuff like 
<?php
$db = new \PDO(
  "mysql:host={SETTINGS['db']['host']};dbname={SETTINGS['db']['name']};charset=utf8",
  SETTINGS['db']['user'],
  SETTINGS['db']['pass'], [\PDO::ATTR_ERRMODE => \PDO::ERRMODE_EXCEPTION
  ]
);
?>

Make sure to hide your settings.ini file on a website with for instance
<?php
<FilesMatch "\.(?:ini|htaccess)$">
  Order allow,deny
  Deny from all
</FilesMatch>
?>

You may want, in some very special cases, to parse multi-dimensional array with N levels in your ini file. Something like setting[data][config][debug] = true will result in an error (expected "=").

Here's a little function to match this, using dots (customizable).
<?php
function parse_ini_file_multi($file, $process_sections = false, $scanner_mode = INI_SCANNER_NORMAL) {
    $explode_str = '.';
    $escape_char = "'";
    // load ini file the normal way
    $data = parse_ini_file($file, $process_sections, $scanner_mode);
    if (!$process_sections) {
        $data = array($data);
    }
    foreach ($data as $section_key => $section) {
        // loop inside the section
        foreach ($section as $key => $value) {
            if (strpos($key, $explode_str)) {
                if (substr($key, 0, 1) !== $escape_char) {
                    // key has a dot. Explode on it, then parse each subkeys
                    // and set value at the right place thanks to references
                    $sub_keys = explode($explode_str, $key);
                    $subs =& $data[$section_key];
                    foreach ($sub_keys as $sub_key) {
                        if (!isset($subs[$sub_key])) {
                            $subs[$sub_key] = [];
                        }
                        $subs =& $subs[$sub_key];
                    }
                    // set the value at the right place
                    $subs = $value;
                    // unset the dotted key, we don't need it anymore
                    unset($data[$section_key][$key]);
                }
                // we have escaped the key, so we keep dots as they are
                else {
                    $new_key = trim($key, $escape_char);
                    $data[$section_key][$new_key] = $value;
                    unset($data[$section_key][$key]);
                }
            }
        }
    }
    if (!$process_sections) {
        $data = $data[0];
    }
    return $data;
}
?>

The following file:
<?php
/*
[normal]
foo = bar
; use quotes to keep your key as it is
'foo.with.dots' = true

[array]
foo[] = 1
foo[] = 2

[dictionary]
foo[debug] = false
foo[path] = /some/path

[multi]
foo.data.config.debug = true
foo.data.password = 123456
*/
?>

will result in:
<?php
parse_ini_file_multi('file.ini', true);

Array
(
    [normal] => Array
        (
            [foo] => bar
            [foo.with.dots] => 1
        )
    [array] => Array
        (
            [foo] => Array
                (
                    [0] => 1
                    [1] => 2
                )
        )
    [dictionary] => Array
        (
            [foo] => Array
                (
                    [debug] => 
                    [path] => /some/path
                )
        )
    [multi] => Array
        (
            [foo] => Array
                (
                    [data] => Array
                        (
                            [config] => Array
                                (
                                    [debug] => 1
                                )
                            [password] => 123456
                        )
                )
        )
)
?>

The documentation states:
Characters ?{}|&~!()^" must not be used anywhere in the key and have a special meaning in the value.

Here's the results of my experiments on what they mean:

; | is used for bitwise OR
three = 2|3

; & is used for bitwise AND
four = 6&5

; ^ is used for bitwise XOR
five = 3^6

; ~ is used for bitwise negate
negative_two = ~1

; () is used for grouping
seven = (8|7)&(6|5)

; ${...} is used for grabbing values from the environment, or previously defined values.
path = ${PATH}
also = ${five}

; ? I have no guess for
; ! I have no guess for

Not mentioned in the documentation, this function acts like include:

"Files are included based on the file path given or, if none is given, the include_path specified. If the file isn't found in the include_path, include will finally check in the calling script's own directory and the current working directory before failing."

(At least for PHP 7; have not checked PHP 5.)

This function for save ini files

<?php
function array_to_ini($array,$out="")
{
    $t="";
    $q=false;
    foreach($array as $c=>$d)
    {
        if(is_array($d))$t.=array_to_ini($d,$c);
        else
        {
            if($c===intval($c))
            {
                if(!empty($out))
                {
                    $t.="\r\n".$out." = \"".$d."\"";
                    if($q!=2)$q=true;
                }
                else $t.="\r\n".$d;
            }
            else
            {    
                $t.="\r\n".$c." = \"".$d."\"";
                $q=2;
            }
        }
    }
    if($q!=true && !empty($out)) return "[".$out."]\r\n".$t;
    if(!empty($out)) return  $t;
    return trim($t);
}

function save_ini_file($array,$file)
{
    $a=array_to_ini($array);
    $ffl=fopen($file,"w");
    fwrite($ffl,$a);
    fclose($ffl);
}
?>

Warning: parse_ini_files cannot cope with values containing the equal sign (=).

The following function supports sections, comments, arrays, and key-value pairs outside of any section.
Beware that similar keys will overwrite one another (unless in different sections).

<?php
function parse_ini ( $filepath ) {
    $ini = file( $filepath );
    if ( count( $ini ) == 0 ) { return array(); }
    $sections = array();
    $values = array();
    $globals = array();
    $i = 0;
    foreach( $ini as $line ){
        $line = trim( $line );
        // Comments
        if ( $line == '' || $line{0} == ';' ) { continue; }
        // Sections
        if ( $line{0} == '[' ) {
            $sections[] = substr( $line, 1, -1 );
            $i++;
            continue;
        }
        // Key-value pair
        list( $key, $value ) = explode( '=', $line, 2 );
        $key = trim( $key );
        $value = trim( $value );
        if ( $i == 0 ) {
            // Array values
            if ( substr( $line, -1, 2 ) == '[]' ) {
                $globals[ $key ][] = $value;
            } else {
                $globals[ $key ] = $value;
            }
        } else {
            // Array values
            if ( substr( $line, -1, 2 ) == '[]' ) {
                $values[ $i - 1 ][ $key ][] = $value;
            } else {
                $values[ $i - 1 ][ $key ] = $value;
            }
        }
    }
    for( $j=0; $j<$i; $j++ ) {
        $result[ $sections[ $j ] ] = $values[ $j ];
    }
    return $result + $globals;
}
?>

Example usage:
<?php
$stores = parse_ini('stores.ini');
print_r( $stores );
?>

An example ini file:
<?php
/*
;Commented line start with ';'
global_value1 = a string value
global_value1 = another string value

; empty lines are discarded
[Section1]
key = value
; whitespace around keys and values is discarded too
otherkey=other value
otherkey=yet another value
; this key-value pair will overwrite the former.
*/
?>

This is a simple (but slightly hackish) way of avoiding the character limitations (in values):

<?php
define('QUOTE', '"');
$test = parse_ini_file('test.ini');

echo "<pre>";
print_r($test);
?>

contents of test.ini:

park yesterday = "I (walked) | {to} " QUOTE"the"QUOTE " park yesterday & saw ~three~ dogs!"

output:

<?php
Array
(
    [park yesterday] => I (walked) | {to} "the" park yesterday & saw ~three~ dogs!
)
?>

.ini files or JSON file format as it is also known as, are very useful format to store stuff in. Especially large arrays. 

Strangely enough there is this nice function to read the file, but no function to write it.

So here is one.

Use it as:  put_ini_file(string $file, array $array)

<?php 
function put_ini_file($file, $array, $i = 0){
  $str="";
  foreach ($array as $k => $v){
    if (is_array($v)){
      $str.=str_repeat(" ",$i*2)."[$k]".PHP_EOL; 
      $str.=put_ini_file("",$v, $i+1);
    }else
      $str.=str_repeat(" ",$i*2)."$k = $v".PHP_EOL; 
  }
 if($file)
    return file_put_contents($file,$str);
  else
    return $str;
}
?>

Just a quick note for all those running into trouble escaping double quotes:

I got around this by "base64_encode()"-ing my content on the way in to the ini file, and "base64_decode()"-ing on the way out.

Because base64 uses the "=" sign, you will have to encapsulate the entire value in double quotes so the line looks like this:

    varname = "TmlhZ2FyYSBGYWxscywgT04="

When base64'd, your strings will retain all \n, \t...etc...  URL's retain everything perfectly :-)

I hope some of you find this useful!

Cheers, Kieran

fix a little bug (here) in function put_ini_file:

function put_ini_file($config, $file, $has_section = false, $write_to_file = true){
    $fileContent = '';
    if(!empty($config)){
        foreach($config as $i=>$v){
            if($has_section){
                $fileContent .= "\n[$i]".PHP_EOL.put_ini_file($v, $file, false, false);
            }
            else{
                if(is_array($v)){
                    foreach($v as $t=>$m){
//--->>> Here                        $fileContent .= "-->$i[$t] = ".(is_numeric($m) ? $m : '"'.$m.'"').PHP_EOL;
                        $fileContent .= "$i"."[] = ".(is_numeric($m) ? $m : '"'.$m.'"').PHP_EOL;
                    }
                }
                else $fileContent .= "$i = ".(is_numeric($v) ? $v : '"'.$v.'"').PHP_EOL;
            }
        }
    }

    if($write_to_file && strlen($fileContent)) return file_put_contents($file, $fileContent, LOCK_EX);
    else return $fileContent;
}

Securing your .ini file:

“ini files are generally treated as plain text by web servers and thus served to browsers if requested. That means for security you must either keep your ini files outside of your docroot or reconfigure your web server to not serve them. Failure to do either of those may introduce a security risk.”

Alternatively, you can save you file as:

stuff.ini.php

add this to the beginning:

;<?php die('go away'); ?>

The semicolon at the beginning is treated as comment, so this line has no effect on the ini file.

Since the file has a .php extension, it will run through the PHP interpreter if you attempt to access this file directly, and the php block will be processed and exit.

The file extension has no ill effect on the parse_ini_file() function, and the .ini part is, of course, a matter of taste.

This core function won't handle ini key[][] = value(s), (multidimensional arrays), so if you need to support that kind of setup you will need to write your own function. one way to do it is to convert all the key = value(s) to array string [key][][]=value(s), then use parse_str() to convert all those [key][][]=value(s) that way you just read the ini file line by line, instead of doing crazy foreach() loops to handle those (multidimensional arrays) in each section, example...

ini file...... config.php

<?php

; This is a sample configuration file
; Comments start with ';', as in php.ini

[first_section]
one = 1
five = 5
animal = BIRD

[second_section]
path = "/usr/local/bin"
URL = "http://www.example.com/~username"

[third_section]
phpversion[] = "5.0"
phpversion[] = "5.1"
phpversion[] = "5.2"
phpversion[] = "5.3"

urls[svn] = "http://svn.php.net"
urls[git] = "http://git.php.net"

[fourth_section]

a[][][] = b
a[][][][] = c
a[test_test][][] = d
test[one][two][three] = true

?>

echo parse_ini_file ( "C:\\services\\www\\docs\\config.php" );

results in...

// PHP Warning:  syntax error, unexpected TC_SECTION, expecting '=' line 27 -> a[][][] = b

Here it simple function that handles (multidimensional arrays) without looping each key[][]= value(s)

<?php

function getIni ( $file, $sections = FALSE )
{
    $return = array ();

    $keeper = array ();

    $config = fopen ( $file, 'r' );

    while ( ! feof ( $config ) )
    { 
        $line = trim ( fgets ( $config, 1024 ) );

        $line = ( $line == '' ) ? ' ' : $line;

        switch ( $line{0} )
        {
            case ' ':
            case '#':
            case '/':
            case ';':
            case '<':
            case '?':

            break;

            case '[':

            if ( $sections )
            {
                $header = 'config[' . trim ( substr ( $line, 1, -1 ) ) . ']';
            }
            else
            {
                $header = 'config';
            }

            break;

            default:

            $kv = array_map ( 'trim', explode ( '=', $line ) );

            $kv[0] = str_replace ( ' ', '+', $kv[0] );

            $kv[1] = str_replace ( ' ', '+', $kv[1] );

            if ( ( $pos = strpos ( $kv[0], '[' ) ) !== FALSE )
            {
                $kv[0] = '[' . substr ( $kv[0], 0, $pos ) . ']' . substr ( $kv[0], $pos );
            }
            else
            {
                $kv[0] = '[' . $kv[0] . ']';
            }

            $bt = strtolower ( $kv[1] );

            if ( in_array ( $bt, array ( 'true', 'false', 'on', 'off' ) ) )
            {
                $kv[1] = ( $bt == 'true' || $bt == 'on' ) ? TRUE : FALSE;
            }

            $keeper[] = $header . $kv[0] . '=' . $kv[1];
        }
    }

    fclose ( $config );

    parse_str ( implode ( '&', $keeper ), $return );

    return $return['config'];
}

// usage...

$sections = TRUE;

print_r ( $config->getIni ( "C:\\services\\www\\docs\\config.php" ),  $sections );

?>