在php中只使用单变量的if条件

if (condition) {
    // 如果条件为真，则执行这里的代码
}

$variable = $_GET['param'];

if ($variable) {
    echo "变量已设置";
} else {
    echo "变量未设置";
}

$name = $_POST['name'];

if (empty($name)) {
    echo "名字不能为空";
} else {
    echo "欢迎，" . htmlspecialchars($name);
}

$age = 25;

if ($age >= 18) {
    echo "成年人";
} else {
    echo "未成年人";
}

if (isset($_GET['param'])) {
    $variable = $_GET['param'];
    echo "变量值: " . $variable;
} else {
    echo "变量未定义";
}

$number = $_POST['number'];

if (is_numeric($number)) {
    $number = intval($number);
    if ($number > 0) {
        echo "正数";
    } else {
        echo "非正数";
    }
} else {
    echo "输入不是数字";
}