Most efficient way to do language file in PHP?

This might work better:

function _L($phrase){
static $_L = array(
    'NO_PHOTO' => 'No photo\'s available',
    'NEW_MEMBER' => 'This user is new'
);

     return (!array_key_exists($phrase,$_L)) ? $phrase : $_L[$phrase];
}

Thats what i use for now. If the language is not found, it will return the phrase, instead of an error.

You should note that an array can contain no more than ~65500 items. Should be enough but well, just saying.

Here's some code that i use to check for the user's language:

<?php
function setSessionLanguageToDefault() {
    $ip=$_SERVER['REMOTE_ADDR'];
    $url='http://api.hostip.info/get_html.php?ip='.$ip;
    $data=file_get_contents($url);
    $s=explode (':',$data);
    $s2=explode('(',$s[1]);

    $country=str_replace(')','',substr($s2[1], 0, 3));

    if ($country=='us') {
        $country='en';
    }

    $country=strtolower(ereg_replace("[^A-Za-z0-9]", "", $country ));
    $_SESSION["_LANGUAGE"]=$country;
}

if (!isset($_SESSION["_LANGUAGE"])) {
    setSessionLanguageToDefault();
}

if (file_exists(APP_DIR.'/language/'.$_SESSION["_LANGUAGE"].'.php')) {
    include(APP_DIR.'/language/'.$_SESSION["_LANGUAGE"].'.php');
} else {
    include(APP_DIR.'/language/'.DEFAULT_LANG.'.php');
}

?>

Its not done yet, but well i think this might help a lot.

It'd probably be best to define a function that handles your language mapping. That way, if you do want to change how it works later, you're not forced to scour hundreds of scripts for cases where you used $lang[...] and replace them with something else.

Something like this would work and would be nice & fast:

function lang($phrase){
    static $lang = array(
        'NO_PHOTO' => 'No photo\'s available',
        'NEW_MEMBER' => 'This user is new'
    );
    return $lang[$phrase];
}

Make sure the array is declared static inside the function so it doesn't get reallocated each time the function is called. This is especially important when $lang is really large.

To use it:

echo lang('NO_PHOTO');

For handling multiple languages, just have this function defined in multiple files (like en.php, fr.php, etc) and require() the appropriate one for the user.