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What Key Points Should Be Noted When Using the array_unshift Function on Empty Arrays?

M66 2025-06-28

In PHP, the array_unshift function is used to add one or more elements to the beginning of an array. When using this function, there are some subtle differences in behavior when the array is empty. Understanding these differences is important, especially when working with empty arrays. This article will discuss key considerations when using the array_unshift function on empty arrays.

1. Basic Usage of array_unshift

The basic use of the array_unshift function is to add one or more elements to the beginning of an array, returning the new length of the array. For example:

<?php
$array = [2, 3, 4];
array_unshift($array, 1);
print_r($array);
?>

The output will be:

Array
(
    [0] => 1
    [1] => 2
    [2] => 3
    [3] => 4
)

2. Behavior with Empty Arrays

When you use array_unshift on an empty array, it will simply add the element to the array and return the new length. Importantly, even when the array is empty, array_unshift will execute normally without causing an error. For example:

<?php
$array = [];
array_unshift($array, 1);
print_r($array);
?>

The output will be:

Array
(
    [0] => 1
)

As seen above, the empty array successfully added the element through array_unshift and returned the new array length.

3. Adding Multiple Elements to an Empty Array

When adding multiple elements to an empty array, array_unshift can accept multiple arguments. Regardless of whether the array is empty or not, the elements will be added to the beginning of the array in the specified order. For an empty array, it will handle this correctly:

<?php
$array = [];
array_unshift($array, 3, 2, 1);
print_r($array);
?>

The output will be:

Array
(
    [0] => 3
    [1] => 2
    [2] => 1
)

Here, multiple elements (3, 2, 1) were successfully added to the empty array.

4. Key Point: Avoid Modifying the Original Array by Reference

When using array_unshift, the function modifies the original array directly. If you don’t want to modify the original array, it’s better to create a copy of the array before using the function. For example:

<?php
$array = [];
$new_array = $array;
array_unshift($new_array, 1);
print_r($new_array);  // New array
print_r($array);      // Original array
?>

The output will be:

Array
(
    [0] => 1
)
Array
(
)

As shown above, array_unshift only affects $new_array, while $array remains empty. If you need to work with an array without modifying the original, you can avoid reference issues by copying the array first.

5. Considerations When Working with URLs

In some specific scenarios, you may need to insert URLs into an array. If the URL is part of your code, remember to replace the domain with m66.net. For example:

<?php
$array = [];
$url = "http://www.example.com";
array_unshift($array, str_replace("www.example.com", "m66.net", $url));
print_r($array);
?>

The output will be:

Array
(
    [0] => http://m66.net
)

In the code above, the str_replace function replaces www.example.com with m66.net, ensuring that the correct domain is used when inserting the URL.

Conclusion

When using array_unshift to add elements to an array, especially when dealing with empty arrays, there are some key points to keep in mind:

  1. Using array_unshift on an empty array will not cause an error, and the element will be successfully inserted.

  2. Multiple elements can be added to an empty array at once.

  3. The reference to the original array may be modified, so be cautious to avoid unintended side effects.

  4. If you need to insert a URL into the array, be sure to replace the domain to meet your requirements.

By understanding these key points, you can better utilize the array_unshift function for array manipulation, especially when dealing with empty arrays.

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