Why can't the first parameter be a string?

M66 2025-06-06

Why can't the first parameter of the array_fill() function in PHP be a string? Why can it be numbers?

In PHP, the array_fill() function is used to fill an array, which accepts three parameters:

Start Index : Specifies the start index of the fill.
Length : Specifies the number of elements to be filled.
Value : Specifies the element value that fills the array.

Typically, the first argument to array_fill() is a number that represents the starting index of the array. If you try to pass a string as the first argument to array_fill() , PHP throws a Warning error indicating that the function cannot handle indexes of string types. Why is this?

1. PHP arrays are indexed by numbers

Arrays in PHP are usually operated with numbers as indexes, especially when filling arrays with array_fill() . array_fill() is designed based on the array fill mechanism of "number index", which fills the array in sequence starting from the specified starting number. The value of the first parameter must be an integer (or a number that can be converted to an integer), because this is the normal behavior of PHP arrays.

If the first parameter is a string, PHP cannot correctly map it to a valid array index because PHP cannot convert a non-numeric string value into a reasonable numeric index. Therefore, it can only handle indexes of numeric types.

2. PHP arrays are ordered, not associative arrays

In PHP, arrays can be either numerical indexes or associative arrays (keys are strings). However, when designing the array_fill() function, the default is to operate the number index array . When you provide a number as an index, PHP knows that the fill should start from that position.

Strings as indexes are usually used in associative arrays, in which each key-value pair consists of string keys and values . array_fill() does not work for this situation, because it does not support starting to fill an associative array with a key as a string at the specified start position.

3. Memory optimization and performance issues

The underlying implementation mechanism of PHP arrays optimizes the performance of digital index arrays, which is more efficient in operating digital indexes. If you use a numeric index, PHP will bind the numeric index directly to the memory location, thus avoiding unnecessary extra calculations. And if you try to use strings as indexes, PHP will require additional memory processing and calculations, resulting in performance degradation.

4. How to use array_fill() correctly

In array_fill() , the first parameter should be an integer. For example, fill an array starting from index 2 with 5 elements:

 $arr = array_fill(2, 5, 'Hello');
print_r($arr);

Output result:

 Array
(
    [2] => Hello
    [3] => Hello
    [4] => Hello
    [5] => Hello
    [6] => Hello
)

In this example, array_fill() fills 5 elements starting from index 2 , each of which is 'Hello' . All indexes are numbers, not strings.

5. Conclusion

The original intention of array_fill() was to provide fill function for arrays with numeric indexes, rather than associative arrays. If you want to fill an associative array, you can use other methods, such as looping in combination with string indexing to assign values:

 $arr = [];
$startIndex = 'a';
for ($i = 0; $i < 5; $i++) {
    $arr[$startIndex] = 'Hello';
    $startIndex++;
}
print_r($arr);

In this example, we fill the array with string keys.

I hope this article can help you better understand the use and limitations of the array_fill() function in PHP. If you have any other questions, please visit our website!

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