Fatal error of php variable variables: Cannot use [] for reading

<code><span><?php</span><span>$nums</span> = <span>array</span>(<span>1</span>, <span>2</span>, <span>3</span>);
<span>$arr_name</span> = <span>'nums'</span>;
<span>$$arr_name</span>[] = <span>4</span>;
print_r(<span>$nums</span>);
<span>?></span></code>

When executing the above program, a Fatal error: Cannot use [] for reading will appear. Because the semantics are ambiguous, change it like this:

<code><span><?php</span><span>$nums</span> = <span>array</span>(<span>1</span>, <span>2</span>, <span>3</span>);
<span>$arr_name</span> = <span>'nums'</span>;
${<span>$arr_name</span>}[] = <span>4</span>;
print_r(<span>$nums</span>);
<span>?></span></code>

The above introduces the Fatal error: Cannot use [] for reading of PHP variable variables, including the relevant content. I hope it will be helpful to friends who are interested in PHP tutorials.