Why Does PHP Issue 'Only Variables Should Be Passed by Reference' Warnings, and How Can They Be Resolved?

Understanding the Strict Standards Warning: "Only Variables Should Be Passed by Reference"

In PHP, the strict standards warning "Only variables should be passed by reference" triggers when an attempt is made to pass a non-variable by reference to a function or method.

Explanation of the Behavior

Consider the following code:

$el = array_shift($instance->find());

This code generates a strict standards warning because $instance->find() returns an array, which is not a variable. When passed as an argument, it attempts to pass the array by reference (using the & operator), whichtriggers the warning.

However, in the following code:

function get_arr(){
    return array(1, 2);
}
$el = array_shift(get_arr());

The strict standards warning is not reported because get_arr() is a function that returns an array. Therefore, it is treated as a variable rather than just an array value.

The Root Cause

The strict standards warning occurs specifically in situations where a method or function is called and its return value is passed by reference. For instance, consider the following code:

class test {
    function test_arr(&$a) {
        var_dump($a);
    }
    function get_arr() {
        return array(1, 2);
    }
}

$t = new test;
$t->test_arr($t->get_arr());

In this code, the strict standards warning is generated because $t->get_arr() returns an array, which is not a variable. However, it is passed by reference to the test_arr method using the & operator.

Resolution

To resolve the strict standards warning, there are two possible approaches:

1. Modify Method Signature: Change the method's signature to not use a reference parameter. For example:

function test_arr($a) {
    var_dump($a);
}

2. Use an Intermediate Variable: Create an intermediate variable, assign the method's return value to it, and pass the intermediate variable by reference. For example:

$inter = $instance->find();
$el = array_shift($inter);

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