PHP $_POST 变量
Reading POST bodies, size limits, and choosing between GET and POST.
Key Points`$_POST` is only populated for form content types.
Reading values
PHP
<?php
// 表单提交到本页，method="post"
$name = trim($_POST["fname"] ?? "");
$age = trim($_POST["age"] ?? "");
if ($name === "" || $age === "") {
echo "请填写完整";
} else {
echo "欢迎 " . htmlspecialchars($name), "\n";
echo "年龄 " . (int)$age, "\n";
}Output
欢迎 Tom
年龄 18$_POST is empty when...
| Content-Type | $_POST | Read with |
|---|---|---|
| application/x-www-form-urlencoded | 已填充 | $_POST |
| multipart/form-data | 已填充（文件进 $_FILES） | $_POST |
| application/json | 空 | php://input + json_decode |
| text/plain | 空 | php://input |
The most common trap with Vue/React frontends — $_POST stays empty for JSON bodies.
PHP
<?php
// 同时兼容表单和 JSON
$ctype = $_SERVER["CONTENT_TYPE"] ?? "";
if (str_contains($ctype, "application/json")) {
$data = json_decode(file_get_contents("php://input"), true) ?? [];
} else {
$data = $_POST;
}
echo "收到字段: " . implode(",", array_keys($data)), "\n";
echo "name = " . ($data["name"] ?? "-"), "\n";Output
收到字段: name,age
name = TomSize limits
| Setting | Default | Effect |
|---|---|---|
| post_max_size | 8M | 超过则 $_POST 整个变空 |
| upload_max_filesize | 2M | 单个上传文件上限 |
| max_input_vars | 1000 | 超过则多余字段被丢弃 |
| memory_limit | 128M | 处理大表单时可能先撞上 |
Exceeding
post_max_size leaves $_POST and $_FILES empty with no error. Users see "please fill in everything" and nobody can find the cause. Check CONTENT_LENGTH on production.PHP
<?php
$len = (int)($_SERVER["CONTENT_LENGTH"] ?? 0);
$max = 8 * 1024 * 1024;
if ($len > $max) {
http_response_code(413);
echo "请求体过大: " . round($len / 1048576, 1) . "MB\n";
exit;
}
echo "大小正常: " . $len . " 字节\n";Output
大小正常: 2048 字节GET or POST
- GET —— queries, paging, sorting, filters; shareable and bookmarkable
- POST —— login, orders, mutations, uploads
- PUT / DELETE —— REST APIs; PHP does not parse them, read
php://input - Never GET a password —— it leaks to logs, history, Referer
PHP
<?php
// RESTful：从请求方法分发
$method = $_SERVER["REQUEST_METHOD"];
$body = [];
if (in_array($method, ["PUT", "PATCH", "DELETE"], true)) {
$body = json_decode(file_get_contents("php://input"), true) ?? [];
}
echo "方法: $method\n";
echo "字段数: " . count($body) . "\n";Output
方法: PUT
字段数: 2取值
PHP
<?php
// 表单提交到本页，method="post"
$name = trim($_POST["fname"] ?? "");
$age = trim($_POST["age"] ?? "");
if ($name === "" || $age === "") {
echo "请填写完整";
} else {
echo "欢迎 " . htmlspecialchars($name), "\n";
echo "年龄 " . (int)$age, "\n";
}Output
欢迎 Tom
年龄 18$_POST 什么时候是空的
| Content-Type | $_POST | 读取方式 |
|---|---|---|
| application/x-www-form-urlencoded | 已填充 | $_POST |
| multipart/form-data | 已填充（文件进 $_FILES） | $_POST |
| application/json | 空 | php://input + json_decode |
| text/plain | 空 | php://input |
用前端框架（Vue、React）发 JSON 时踩这个坑最多——后端 $_POST 永远是空的。
PHP
<?php
// 同时兼容表单和 JSON
$ctype = $_SERVER["CONTENT_TYPE"] ?? "";
if (str_contains($ctype, "application/json")) {
$data = json_decode(file_get_contents("php://input"), true) ?? [];
} else {
$data = $_POST;
}
echo "收到字段: " . implode(",", array_keys($data)), "\n";
echo "name = " . ($data["name"] ?? "-"), "\n";Output
收到字段: name,age
name = Tom大小限制
| 配置项 | 默认值 | 影响 |
|---|---|---|
| post_max_size | 8M | 超过则 $_POST 整个变空 |
| upload_max_filesize | 2M | 单个上传文件上限 |
| max_input_vars | 1000 | 超过则多余字段被丢弃 |
| memory_limit | 128M | 处理大表单时可能先撞上 |
post_max_size 超限时 $_POST 和 $_FILES 都是空的，而且不会报错。用户看到的是「请填写完整」，排查半天找不到原因。上线前一定要检查 $_SERVER["CONTENT_LENGTH"]。PHP
<?php
$len = (int)($_SERVER["CONTENT_LENGTH"] ?? 0);
$max = 8 * 1024 * 1024;
if ($len > $max) {
http_response_code(413);
echo "请求体过大: " . round($len / 1048576, 1) . "MB\n";
exit;
}
echo "大小正常: " . $len . " 字节\n";Output
大小正常: 2048 字节GET 还是 POST
- 用 GET —— 查询、分页、排序、筛选，结果可分享可收藏
- 用 POST —— 登录、下单、修改数据、上传文件
- 用 PUT / DELETE —— RESTful API，PHP 不自动解析，需读
php://input - 绝不用 GET 传密码 —— 会留在日志、历史、Referer
PHP
<?php
// RESTful：从请求方法分发
$method = $_SERVER["REQUEST_METHOD"];
$body = [];
if (in_array($method, ["PUT", "PATCH", "DELETE"], true)) {
$body = json_decode(file_get_contents("php://input"), true) ?? [];
}
echo "方法: $method\n";
echo "字段数: " . count($body) . "\n";Output
方法: PUT
字段数: 2