如何使用PHP生成并发进程？

<?php
$processes = 5; // 需要生成的进程数量

for ($i = 0; $i < $processes; $i++) {
    $pid = pcntl_fork();
    
    if ($pid == -1) {
        // 创建进程失败
        exit("Failed to fork process.");
    } elseif ($pid == 0) {
        // 子进程代码
        echo "Child process " . getmypid() . " started.\n";
        
        // 在这里执行子进程需要完成的任务
        
        echo "Child process " . getmypid() . " finished.\n";
        exit(); // 子进程执行完任务后退出
    } else {
        // 父进程代码
        echo "Parent process " . getmypid() . " created child process " . $pid . ".\n";
    }
}

// 等待所有子进程结束
while (pcntl_waitpid(0, $status) != -1) {
    $status = pcntl_wexitstatus($status);
    echo "Child process " . $status . " exited.\n";
}
?>