Issue running Sentry on php5.3: init() undefined

I would like to use Sentry on a php5.3 application (stuck there for a while, until i finish porting the code to PHP8+). I get this error message:

Fatal error: Call to undefined function Sentry\init() in /var/www/html/membres/test_sentry.php on line 9

Line 8 and 9:

require '../vendor/autoload.php';
\Sentry\init(array('dsn' => '' ));

Here is my composer.json

    "require": {
        "sentry/sentry": "^1.0"
    "config": {
        "platform": {
            "php": "5.3"

Does this mean Sentry cannot be run on php5.3?