Why Does PHP Throw a \'Can\'t Use Function Return Value in Write Context\' Error?

PHP Error Analysis: Understanding "Can't Use Function Return Value in Write Context"

The error message "Can't use function return value in write context" indicates that PHP is unable to assign a function's return value to a variable or write it to a file. This error commonly occurs when trying to evaluate a function's return directly in a conditional statement.

In the provided example:

The error appears on line 48, which reads:

if (isset($_POST('sms_code') == TRUE ) {...}

Here, PHP is attempting to evaluate the truthiness of isset($_POST('sms_code')) and assign the result to TRUE. However, isset() is a language construct, not a function, and its return value cannot be used in this context.

Solution:

To resolve this error, directly assign the return value of isset() to a boolean variable:

$sms_code_exists = isset($_POST('sms_code'));

if ($sms_code_exists) {...}

Additional Note:

This error can also occur when using the empty language construct on a function return value. For example:

!empty(trim($someText)) and doSomething()

In such cases, it is correct to evaluate the return value of trim() using the comparison operator:

trim($someText) !== '' and doSomething()

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