How can I detect if the user is on localhost in PHP?

As a complement, as a function...

function isLocalhost($whitelist = ['127.0.0.1', '::1']) {
    return in_array($_SERVER['REMOTE_ADDR'], $whitelist);
}

You can also use $_SERVER['REMOTE_ADDR'] for which IP address of the client requesting is given by the web server.

$whitelist = array(
    '127.0.0.1',
    '::1'
);

if(!in_array($_SERVER['REMOTE_ADDR'], $whitelist)){
    // not valid
}

Newer OS users (Win 7, 8) may also find it necessary to include an IPV6-format remote address in their whitelist array:

$whitelist = array('127.0.0.1', "::1");

if(!in_array($_SERVER['REMOTE_ADDR'], $whitelist)){
    // not valid
}

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