Using `$started || start()`;

started.php

<?php

$started = false;

function start() {
    global $started;

    if ($started) {
        print("Already started\n");
        return;
    }

    print("Starting\n");
    $started = true;
}

function run() {
    global $started;

    print("Running (is " . ($started ? 1 : 0) . ")\n");

    $started || start();

    print("Now is " . ($started ? 1 : 0) . "\n");
}

run();