What problems will occur when using negative numbers as the $start_index parameter?

M66 2025-06-06

According to the official PHP documentation, the $start_index of array_fill() can accept negative numbers. This means that if a negative number is passed as $start_index , PHP treats it as the position that starts from the end of the array.

For example, when $start_index is -1 , it points to the last element of the array. When $start_index is -2 , it points to the penultimate element, and so on.

Sample code

 $array = array_fill(-3, 5, 'PHP');
print_r($array);

In this example, we pass in a negative number -3 , indicating that the fill starts from the third last position of the array. The result will be:

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

Although we passed -3 as the start index, the array_fill() function does not generate an error or warning, but successfully fills the array, and the start position will be automatically adjusted according to the negative index. The final array can still be successfully created starting from index 0.

Problems caused by negative indexes

Although using a negative number as the $start_index parameter is valid in some cases, it can also lead to unexpected results or difficult-to-understand behavior. Especially when the initial index of an array is not starting from zero, or the array does not have enough elements, negative indexes can cause some confusion.

1. Fill out of array boundary

If the $num parameter of array_fill() is large and $start_index is a negative number, it may attempt to access a location of the array, which does not exist. For example:

 $array = array_fill(-10, 5, 'PHP');
print_r($array);

In this case, -10 will be considered as the 10th position ahead from the end of the array. If the original array is empty or not enough to support padding from negative indexes, it may result in unexpected results. Although PHP does not report an error, its behavior will cause you to fail to get the expected results.

2. Confusion in array indexes

The behavior of negative indexes can cause confusion when the original index of an array is not started from zero. For example, if there is already an array and passed it into array_fill() , if $start_index is negative, the index of the resulting array will become unpredictable.

How to avoid problems

To avoid the above problems, it is recommended to double-check the passed $start_index when using array_fill() . If you need to start padding from the end of the array, you can explicitly use negative indexes, but make sure that negative indexes do not cause padding to exceed the boundaries of the array, especially when dealing with dynamic data.

In addition, if you do not want the fill behavior to exceed the expected range, you can first check whether $start_index is reasonable, or use functions such as array_slice() to limit the size of the array before filling.

array_fill