这个错误在 PHP 中是什么意思？

Question

What is this?

This is a number of answers about warnings, errors and notices you might encounter while programming PHP and have no clue how to fix. This is also a Community Wiki, so everyone is invited to participate in adding to and maintaining this list.

Why is this?

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 his/her 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 can not 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.

What should I do here?

If your question has been marked as a duplicate of this, please find your error message below and apply the fix to your code. The answers usually contain further links to investigate in case it shouldn't be clear from the general answer alone.

If you want to contribute, please add your "favorite" error message, warning or notice, one per answer, a short description what it means (even if it is only highlighting terms to their manual page), a possible solution or debugging approach and a listing of existing Q&A that are of value. Also, feel free to improve any existing answers.

The List

• 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 a.k.a.
  Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, boolean given (or similar variations)
• 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'
• 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: Call to a member function ... on a non-object
• 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() '
• Fatal error: Using $this when not in object context
• Notice: Array to string conversion
• Notice: Trying to get property of non-object error
• Notice: Undefined variable
• Notice: Undefined Index
• Notice: Undefined offset XXX [Reference]
• Notice: Uninitialized string offset: XXX
• Notice: Use of undefined constant XXX - assumed '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

Also see

• Reference - What does this symbol mean in PHP?

转载于:https://stackoverflow.com/questions/12769982/reference-what-does-this-error-mean-in-php

Answer 1

Fatal error: Call to a member function ... on a non-object

Happens with code similar to xyz->method() where xyz is not an object and therefore that method can not be called.

This is a fatal error which will stop the script (forward compatibility notice: It will become a catchable error starting with PHP 7).

Most often this is a sign that the code has missing checks for error conditions. Validate that an object is actually an object before calling its methods.

A typical example would be

// ... some code using PDO
$statement = $pdo->prepare('invalid query', ...);
$statement->execute(...);

In the example above, the query cannot be prepared and prepare() will assign false to $statement. Trying to call the execute() method will then result in the Fatal Error because false is a "non-object" because the value is a boolean.

Figure out why your function returned a boolean instead of an object. For example, check the $pdo object for the last error that occurred. Details on how to debug this will depend on how errors are handled for the particular function/object/class in question.

If even the ->prepare is failing then your $pdo database handle object didn't get passed into the current scope. Find where it got defined. Then pass it as a parameter, store it as property, or share it via the global scope.

Another problem may be conditionally creating an object and then trying to call a method outside that conditional block. For example

if ($someCondition) {
    $myObj = new MyObj();
}
// ...
$myObj->someMethod();

By attempting to execute the method outside the conditional block, your object may not be defined.

Related Questions:

• Call to a member function on a non-object
• List all PHP "Fatal error: Call to a member function ... on a non-object" Questions on Stackoverflow