The difference between PHP single quotes and double quotes

<code><span><?php</span><span>$str</span> = <span>'\',\\,\n,\t,\r,...'</span>;
<span>echo</span><span>$str</span>;
<span>?></span>
输出结果
<span>',\,\n,\t,\r,...</span></code>

<code><span><?php</span><span>$str</span> = <span>"\",\\,\n,\t,\r,..."</span>;
<span>echo</span><span>$str</span>;
<span>?></span>
输出结果：
<span>",\, ,  , ,...</span></code>

<code><span><?php</span><span>$a</span> = <span>10</span>;
<span>echo</span><span>'a的值是$a'</span>;
<span>echo</span><span>'<br />'</span><span>echo</span><span>"a的值是$a"</span>;
<span>?></span>
输出结果：
a的值是<span>$a</span>
a的值是<span>10</span></code>