Why Does PHP Issue 'Strict Standards: Only variables should be passed by reference' When Using array_shift()?

Understanding Reference Passing in PHP: Strict Standards and Array_Shift()

In PHP, strict standards mode can issue warnings when passing non-variable values by reference. A common example of this is when using the array_shift() function.

When Does the Warning Occur?

The "Strict standards: Only variables should be passed by reference" warning appears when array_shift() is used with a non-variable value passed as an argument. For instance:

$instance = new MyClass();
$el = array_shift($instance->find(...)); // Warning

In contrast, when array_shift() is called with a variable containing an array, no warning is generated:

function get_arr() {
    return [1, 2];
}
$el = array_shift(get_arr()); // No warning

Non-Intuitive Behavior

The warning can be confusing because array_shift() is a function that returns an array value. However, under strict mode, PHP considers the return value of array_shift() as a non-variable.

Resolving the Warning

To resolve the warning in strict mode, there are two options:

1. Modify the Method Signature: Change the signature of the method returning the array to avoid using a reference.
2. Use an Intermediate Variable: Assign the array returned by the method to an intermediate variable before passing it to array_shift().

For example:

// Modify Method Signature
function get_arr() {
    return [1, 2];
}
$instance = new MyClass();
$el = array_shift($instance->get_arr());

// Use Intermediate Variable
$el = array_shift($instance->get_arr() ?: []);

The above is the detailed content of Why Does PHP Issue 'Strict Standards: Only variables should be passed by reference' When Using array_shift()?. For more information, please follow other related articles on the PHP Chinese website!