这是什么?
这是一些关于在编程PHP时可能遇到的警告、错误和注意事项的答案，而不知道如何修复它们。这也是一个社区维基，所以每个人都被邀请加入和维护这个列表。
为什么会这样?
Questions like "Headers already sent" or "Calling a member of a non-object" pop up frequently on Stack Overflow. The root cause of those questions is always the same. So the answers to those questions typically repeat them and then show the OP which line to change in their particular case. These answers do not add any value to the site because they only apply to the OP's particular code. Other users having the same error cannot easily read the solution out of it because they are too localized. That is sad because once you understood the root cause, fixing the error is trivial. Hence, this list tries to explain the solution in a general way to apply.
我该怎么做呢?
如果您的问题被标记为此问题的副本，请在下面找到您的错误消息并将修复应用于您的代码。答案通常包含进一步的调查链接，以防仅从一般答案中不清楚。
如果您想投稿，请添加您“最喜欢的”错误消息、警告或通知，每个答案一条，简短描述它的含义(即使它只是突出显示手册页的术语)，可能的解决方案或调试方法，以及现有的有价值的问答列表。此外，请随意改进任何现有的答案。
列表
Nothing is seen. The page is empty and white. (also known as White Page/Screen Of Death)
Code doesn't run/what looks like parts of my PHP code are output
Warning: Cannot modify header information - headers already sent
Warning: mysql_fetch_array() expects parameter 1 to be resource, boolean given a.k.a.
Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource
Warning: [function] expects parameter 1 to be resource, boolean given
Warning: [function]: failed to open stream: [reason]
Warning: open_basedir restriction in effect
Warning: Division by zero
Warning: Illegal string offset 'XXX'
Warning: count(): Parameter must be an array or an object that implements Countable
Parse error: syntax error, unexpected '['
Parse error: syntax error, unexpected T_XXX
Parse error: syntax error, unexpected T_ENCAPSED_AND_WHITESPACE
Parse error: syntax error, unexpected T_PAAMAYIM_NEKUDOTAYIM
Parse error: syntax error, unexpected 'require_once' (T_REQUIRE_ONCE), expecting function (T_FUNCTION)
Parse error: syntax error, unexpected T_VARIABLE
Fatal error: Allowed memory size of XXX bytes exhausted (tried to allocate XXX bytes)
Fatal error: Maximum execution time of XX seconds exceeded
Fatal error: Call to a member function ... on a non-object or null
Fatal Error: Call to Undefined function XXX
Fatal Error: Cannot redeclare XXX
Fatal error: Can't use function return value in write context
Fatal error: Declaration of AAA::BBB() must be compatible with that of CCC::BBB()'
Return type of AAA::BBB() should either be compatible with CCC::BBB(), or the #[\ReturnTypeWillChange] attribute should be used
Fatal error: Using $this when not in object context
Fatal error: Object of class Closure could not be converted to string
Fatal error: Undefined class constant
Fatal error: Uncaught TypeError: Argument #n must be of type x, y given
Notice: Array to string conversion (< PHP 8.0) or Warning: Array to string conversion (>= PHP 8.0)
Notice: Trying to get property of non-object error
Notice: Undefined variable or property
"Notice: Undefined Index", or "Warning: Undefined array key"
Notice: Undefined offset XXX [Reference]
Notice: Uninitialized string offset: XXX
Notice: Use of undefined constant XXX - assumed 'XXX' / Error: Undefined constant XXX
MySQL: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ... at line ...
Strict Standards: Non-static method [<class>::<method>] should not be called statically
Warning: function expects parameter X to be boolean/string/integer
HTTP Error 500 - Internal server error
Deprecated: Arrays and strings offset access syntax with curly braces is deprecated
还看到:
这个符号在PHP中是什么意思?
注意:使用未定义的常量XXX -假设“XXX”
或者，在PHP 7.2或更高版本中:
警告:使用未定义的常量XXX -假设“XXX”(这将在未来的PHP版本中抛出错误)
或者，在PHP 8.0或更高版本中:
错误:未定义常量XXX
这种情况发生在代码中使用令牌并且看起来是常量，但没有定义该名称的常量时。
引起此通知的最常见原因之一是没有引用用作关联数组键的字符串。
例如:
// Wrong
echo $array[key];
// Right
echo $array['key'];
另一个常见的原因是变量名前缺少$ (dollar)符号:
// Wrong
echo varName;
// Right
echo $varName;
或者你可能拼写错了其他常数或关键字:
// Wrong
$foo = fasle;
// Right
$foo = false;
当您试图访问由该库定义的常量时，这也可能表明缺少所需的PHP扩展或库。
相关问题:
PHP错误消息“注意:使用未定义的常量”是什么意思?
警告:不能修改报头信息-报头已经发送
当您的脚本试图向客户端发送HTTP报头，但之前已经有输出，导致报头已经发送到客户端时发生。
这是一个E_WARNING，它不会停止脚本。
一个典型的例子是这样的模板文件:
<html>
<?php session_start(); ?>
<head><title>My Page</title>
</html>
...
session_start()函数将尝试向客户端发送带有会话cookie的报头。但是PHP在将<html>元素写入输出流时已经发送了头文件。您必须将session_start()移到顶部。
您可以通过遍历代码触发Warning之前的行并检查它输出的位置来解决这个问题。移动任何头发送代码之前的代码。
一个经常被忽略的输出是PHP关闭?>后的新行。当?>是文件中的最后一个内容时，省略它被认为是一种标准实践。同样，出现此警告的另一个常见原因是当开头<?php前面有一个空格，行，或者不可见的字符，导致web服务器发送报头和空格/换行，这样当php开始解析时就不能提交任何报头。
如果您的文件有多个<?php……?>代码块，它们之间不应该有任何空格。(注意:如果你有自动构造的代码，你可能有多个块)
还要确保在代码中没有任何字节顺序标记，例如当脚本的编码是UTF-8和BOM时。
相关问题:
头文件已经被PHP发送
所有PHP“头已经发送”的问题在Stackoverflow
字节顺序标记
什么PHP函数可以创建输出?
已弃用:不支持使用花括号的数组和字符串偏移访问语法
在PHP 7.4.0之前，字符串偏移量和数组元素可以通过花括号{}访问:
$string = 'abc';
echo $string{0}; // a
$array = [1, 2, 3];
echo $array{0}; // 1
自PHP 7.4.0起已弃用，并生成一个警告:
已弃用:不支持使用花括号的数组和字符串偏移访问语法
你必须使用方括号[]来访问字符串偏移量和数组元素:
$string = 'abc';
echo $string[0]; // a
$array = [1, 2, 3];
echo $array[0]; // 1
此更改的RFC链接到一个PHP脚本，该脚本试图机械地修复此问题。
致命错误:调用未定义的函数XXX
在尝试调用尚未定义的函数时发生。常见的原因包括缺少扩展和包含，有条件的函数声明，函数声明中的函数或简单的拼写错误。
例1 -条件函数声明
$someCondition = false;
if ($someCondition === true) {
function fn() {
return 1;
}
}
echo fn(); // triggers error
在这种情况下，fn()将永远不会被声明，因为$someCondition不为真。
例2 -函数声明中的函数
function createFn()
{
function fn() {
return 1;
}
}
echo fn(); // triggers error
在这种情况下，只有在调用createFn()时才声明fn。注意，后续调用createFn()将触发一个关于重新声明现有函数的错误。
在PHP内置函数中也可以看到这种情况。尝试在官方手册中搜索该函数，并检查它属于哪个“扩展”(PHP模块)，以及哪些版本的PHP支持它。
如果缺少扩展，请安装该扩展并在php.ini中启用它。请参阅PHP手册中的安装说明，以了解函数在扩展中的显示。你也可以使用你的包管理器(例如Debian或Ubuntu中的apt, Red Hat或CentOS中的yum)，或共享托管环境中的控制面板来启用或安装扩展。
如果该函数是在您正在使用的PHP的新版本中引入的，那么您可以在手册或评论部分中找到替代实现的链接。如果它已从PHP中删除，请查找有关原因的信息，因为可能不再需要它了。
如果缺少include，请确保在调用函数之前包含声明该函数的文件。
如果出现错别字，请改正错别字。
相关问题:
https://stackoverflow.com/search?q=Fatal+error%3A+Call+to+undefined+function
mysql_connect(): user 'name'@'host'拒绝访问
当您连接到无效或缺少凭据(用户名/密码)的MySQL/MariaDB服务器时，会出现此警告。所以这通常不是代码问题，而是服务器配置问题。
See the manual page on mysql_connect("localhost", "user", "pw") for examples.
Check that you actually used a $username and $password.
It's uncommon that you gain access using no password - which is what happened when the Warning: said (using password: NO).
Only the local test server usually allows to connect with username root, no password, and the test database name.
You can test if they're really correct using the command line client:
mysql --user="username" --password="password" testdb
Username and password are case-sensitive and whitespace is not ignored. If your password contains meta characters like $, escape them, or put the password in single quotes.
Most shared hosting providers predeclare mysql accounts in relation to the unix user account (sometimes just prefixes or extra numeric suffixes). See the docs for a pattern or documentation, and CPanel or whatever interface for setting a password.
See the MySQL manual on Adding user accounts using the command line. When connected as admin user you can issue a query like:
CREATE USER 'username'@'localhost' IDENTIFIED BY 'newpassword';
Or use Adminer or WorkBench or any other graphical tool to create, check or correct account details.
If you can't fix your credentials, then asking the internet to "please help" will have no effect. Only you and your hosting provider have permissions and sufficient access to diagnose and fix things.
Verify that you could reach the database server, using the host name given by your provider:
ping dbserver.hoster.example.net
Check this from a SSH console directly on your webserver. Testing from your local development client to your shared hosting server is rarely meaningful.
Often you just want the server name to be "localhost", which normally utilizes a local named socket when available. Othertimes you can try "127.0.0.1" as fallback.
Should your MySQL/MariaDB server listen on a different port, then use "servername:3306".
If that fails, then there's a perhaps a firewall issue. (Off-topic, not a programming question. No remote guess-helping possible.)
When using constants like e.g. DB_USER or DB_PASSWORD, check that they're actually defined.
If you get a "Warning: Access defined for 'DB_USER'@'host'" and a "Notice: use of undefined constant 'DB_PASS'", then that's your problem.
Verify that your e.g. xy/db-config.php was actually included and whatelse.
Check for correctly set GRANT permissions.
It's not sufficient to have a username+password pair.
Each MySQL/MariaDB account can have an attached set of permissions.
Those can restrict which databases you are allowed to connect to, from which client/server the connection may originate from, and which queries are permitted.
The "Access denied" warning thus may as well show up for mysql_query calls, if you don't have permissions to SELECT from a specific table, or INSERT/UPDATE, and more commonly DELETE anything.
You can adapt account permissions when connected per command line client using the admin account with a query like:
GRANT ALL ON yourdb.* TO 'username'@'localhost';
If the warning shows up first with Warning: mysql_query(): Access denied for user ''@'localhost' then you may have a php.ini-preconfigured account/password pair.
Check that mysql.default_user= and mysql.default_password= have meaningful values.
Oftentimes this is a provider-configuration. So contact their support for mismatches.
Find the documentation of your shared hosting provider:
e.g. HostGator, GoDaddy, 1and1, DigitalOcean, BlueHost, DreamHost, MediaTemple, ixWebhosting, lunarhosting, or just google yours´.
Else consult your webhosting provider through their support channels.
Note that you may also have depleted the available connection pool. You'll get access denied warnings for too many concurrent connections. (You have to investigate the setup. That's an off-topic server configuration issue, not a programming question.)
Your libmysql client version may not be compatible with the database server. Normally MySQL and MariaDB servers can be reached with PHPs compiled in driver. If you have a custom setup, or an outdated PHP version, and a much newer database server, or significantly outdated one - then the version mismatch may prevent connections. (No, you have to investigate yourself. Nobody can guess your setup).
更多的引用:
mysql服务器错误:mysql用户无法访问root用户
mysql_connect():拒绝访问
mysql_select_db()拒绝访问用户" @'localhost'(使用密码:NO)
PHPMyAdmin拒绝用户“root”@“localhost”的访问
顺便说一句，你可能不想再使用mysql_*函数了。新来者经常迁移到mysqli，然而这也一样乏味。相反，阅读PDO和准备好的语句。
$db =新PDO("mysql:主机=localhost;dbname=testdb"， "用户名"，"密码");
