[用array_fill填充负索引数组会发生什么？行为和结果分析

gitbox 2025-09-30

1. 基本使用示例

<span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">array_fill</span></span><span>(</span><span><span class="hljs-number">0</span></span><span>, </span><span><span class="hljs-number">3</span></span><span>, </span><span><span class="hljs-string">'A'</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">print_r</span></span><span>(</span><span><span class="hljs-variable">$result</span></span><span>);
</span></span>

输出结果：

<span><span><span class="hljs-title function_ invoke__">Array</span></span><span>
(
    [</span><span><span class="hljs-number">0</span></span><span>] =&gt; A
    [</span><span><span class="hljs-number">1</span></span><span>] =&gt; A
    [</span><span><span class="hljs-number">2</span></span><span>] =&gt; A
)
</span></span>

这与预期一致，从索引 0 开始填充 3 个元素。

2. 使用负数作为起始索引

当 $start_index 为负数时，PHP 并不会抛出错误，而是直接以该负数作为键名填充数组。

示例：

<span><span><span class="hljs-variable">$result</span></span><span> = </span><span><span class="hljs-title function_ invoke__">array_fill</span></span><span>(-</span><span><span class="hljs-number">3</span></span><span>, </span><span><span class="hljs-number">3</span></span><span>, </span><span><span class="hljs-string">'B'</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">print_r</span></span><span>(</span><span><span class="hljs-variable">$result</span></span><span>);
</span></span>

输出结果：

<span><span><span class="hljs-title function_ invoke__">Array</span></span><span>
(
    [-</span><span><span class="hljs-number">3</span></span><span>] =&gt; B
    [-</span><span><span class="hljs-number">2</span></span><span>] =&gt; B
    [-</span><span><span class="hljs-number">1</span></span><span>] =&gt; B
)
</span></span>

可以看到，数组从 -3 开始，依次填充到 -1。
这说明 负数索引在 PHP 中是允许的，并且能按照正常的整数键规则向上递增。

3. 与正数索引混合使用的影响

我们再看一个混合场景：

<span><span><span class="hljs-variable">$result1</span></span><span> = </span><span><span class="hljs-title function_ invoke__">array_fill</span></span><span>(-</span><span><span class="hljs-number">2</span></span><span>, </span><span><span class="hljs-number">2</span></span><span>, </span><span><span class="hljs-string">'X'</span></span><span>);
</span><span><span class="hljs-variable">$result2</span></span><span> = </span><span><span class="hljs-title function_ invoke__">array_fill</span></span><span>(</span><span><span class="hljs-number">3</span></span><span>, </span><span><span class="hljs-number">2</span></span><span>, </span><span><span class="hljs-string">'Y'</span></span><span>);
</span><span><span class="hljs-variable">$merged</span></span><span> = </span><span><span class="hljs-variable">$result1</span></span><span> + </span><span><span class="hljs-variable">$result2</span></span><span>; </span><span><span class="hljs-comment">// 使用数组相加</span></span><span>
</span><span><span class="hljs-title function_ invoke__">print_r</span></span><span>(</span><span><span class="hljs-variable">$merged</span></span><span>);
</span></span>

输出结果：

<span><span><span class="hljs-title function_ invoke__">Array</span></span><span>
(
    [-</span><span><span class="hljs-number">2</span></span><span>] =&gt; X
    [-</span><span><span class="hljs-number">1</span></span><span>] =&gt; X
    [</span><span><span class="hljs-number">3</span></span><span>] =&gt; Y
    [</span><span><span class="hljs-number">4</span></span><span>] =&gt; Y
)
</span></span>

这说明即使负数和正数索引同时存在，它们依然保持各自独立的键名空间，不会发生冲突。

4. 注意事项

数组键允许负数
在 PHP 中，数组键既可以是正数，也可以是负数，只要是整数即可。

与 array_merge 的区别
使用 array_merge 时，数值键会被重新索引，而负数键也会被重新整理为从 0 开始的正整数。
示例：

<span><span><span class="hljs-variable">$a</span></span><span> = </span><span><span class="hljs-title function_ invoke__">array_fill</span></span><span>(-</span><span><span class="hljs-number">2</span></span><span>, </span><span><span class="hljs-number">3</span></span><span>, </span><span><span class="hljs-string">'Z'</span></span><span>);
</span><span><span class="hljs-variable">$b</span></span><span> = </span><span><span class="hljs-title function_ invoke__">array_fill</span></span><span>(</span><span><span class="hljs-number">5</span></span><span>, </span><span><span class="hljs-number">2</span></span><span>, </span><span><span class="hljs-string">'W'</span></span><span>);
</span><span><span class="hljs-variable">$merged</span></span><span> = </span><span><span class="hljs-title function_ invoke__">array_merge</span></span><span>(</span><span><span class="hljs-variable">$a</span></span><span>, </span><span><span class="hljs-variable">$b</span></span><span>);
</span><span><span class="hljs-title function_ invoke__">print_r</span></span><span>(</span><span><span class="hljs-variable">$merged</span></span><span>);
</span></span>

输出结果：

<span><span><span class="hljs-title function_ invoke__">Array</span></span><span>
(
    [</span><span><span class="hljs-number">0</span></span><span>] =&gt; Z
    [</span><span><span class="hljs-number">1</span></span><span>] =&gt; Z
    [</span><span><span class="hljs-number">2</span></span><span>] =&gt; Z
    [</span><span><span class="hljs-number">3</span></span><span>] =&gt; W
    [</span><span><span class="hljs-number">4</span></span><span>] =&gt; W
)
</span></span>

可见，负数索引在合并过程中会丢失原有的键值，转而重新编号。