Use the Requests lib to wait for the built-in PHP server to start up

wait-for-php-server-start.php

<?php

$process = new Process( sprintf( 'php -S localhost:8080 -t %s', __DIR__ ) );
$process->start();

$start = time();

// Wait for the server to start up (5 seconds max).
do {
    try {
        Requests::get( 'http://localhost:8080' );
    } catch ( Requests_Exception $e ) {
        continue;
    }
    break;
} while ( time() - $start < 5 );

Requests::get( 'http://localhost:8080/test.php' );