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?
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?
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?
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。
$db =新PDO("mysql:主机=localhost;dbname=testdb"， "用户名"，"密码");
严格标准:非静态方法[<class>::<method>]不应该被静态调用
当您试图在类上调用静态的非静态方法时发生，并且您在error_reporting()设置中也有E_STRICT标志。
例子:
class HTML {
public function br() {
echo '<br>';
}
}
HTML::br()或$ HTML::br()
实际上，您可以通过不向error_reporting()添加E_STRICT来避免此错误，例如
error_reporting(E_ALL & ~E_STRICT);
因为对于PHP 5.4.0及以上版本，E_STRICT包含在E_ALL [ref]中。但这并不可取。解决方案是将你想要的静态函数定义为实际的静态函数:
public static function br() {
echo '<br>';
}
或者按常规调用函数:
$html = new HTML();
$html->br();
相关问题:
我如何解决“非静态方法xxx:xxx()不应该在PHP 5.4中被静态调用?”
什么也看不见。页面是空白的，白色的。
也被称为白页死亡或白屏幕死亡。当错误报告被关闭并且发生致命错误(通常是语法错误)时，就会发生这种情况。
如果启用了错误日志记录，您将在错误日志中找到具体的错误消息。这通常在一个名为“php_errors.log”的文件中，要么在一个中心位置(例如在许多Linux环境中是/var/log/apache2)，要么在脚本本身的目录中(有时在共享主机环境中使用)。
有时，临时启用错误显示可能更直接。然后，白页将显示错误消息。要小心，因为这些错误对访问网站的每个人都是可见的。
这可以通过在脚本顶部添加以下PHP代码轻松完成:
ini_set('display_errors', 1); error_reporting(~0);
代码将打开错误显示并将报告设置为最高级别。
由于ini_set()是在运行时执行的，它对解析/语法错误没有影响。这些错误将出现在日志中。如果你想在输出中显示它们(例如在浏览器中)，你必须将display_startup_errors指令设置为true。在php.ini或.htaccess或任何其他在运行时之前影响配置的方法中执行此操作。
您可以使用相同的方法设置log_errors和error_log指令来选择您自己的日志文件位置。
查看日志或使用显示，您将得到更好的错误消息和脚本停止的代码行。
相关问题:
PHP的白屏死机
白屏死机!
PHP不显示错误消息
PHP在错误时释放500 -在哪里有文档?
如何在PHP中获得有用的错误消息?
关于Stackoverflow的所有PHP“死亡白页”问题
相关的错误:
解析错误:语法错误，意外的T_XXX
致命错误:调用成员函数…在一个非物体上
代码不运行/我的PHP代码的某些部分被输出
