What is the proper value type of the new value when using ini_set()?

On php.net the syntax looks like this:

string ini_set(string $varname, string $newvalue)

All parameters should be a string. On the php.ini file all booleans shown as On or Off. The following solution should be the most proper solution:

ini_set('display_errors', 'On');
ini_set('display_errors', 'Off');

On the documentation of the configuration file you can find the following part:

Boolean values can be set to either:
true, on, yes or false, off, no, none

http://php.net/manual/en/configuration.file.php

On ini_get the return value is a string. The documentation says:

A boolean ini value of off will be returned as an empty string or "0" while a boolean ini value of on will be returned as "1". The function can also return the literal string of INI value.

http://php.net/manual/en/function.ini-get.php

The return value of ini_get and the value for ini_set have to be a string!

Referring : http://php.net/manual/en/function.ini-set.php

string ini_set ( string $varname , string $newvalue )

So you have to use a string for newValue

eg:

<?php
echo ini_get('display_errors');

if (!ini_get('display_errors')) {
    ini_set('display_errors', '1');
}

echo ini_get('display_errors');
?>