PHP 変数 - 一重引用符はコンパイルされず、二重引用符はコンパイルされます

<?<span>php
</span><span>header</span>("Content-type:text/html;charset='utf8'"<span>);
</span><span>error_reporting</span>(<span>E_ALL</span><span>);

</span><span>$sTemp</span> = 'Hello World!<br>'<span>;
</span><span>echo</span> "直接打印输出:".<span>$sTemp</span><span>;
</span><span>echo</span> "在单引号里打印输出:".'$sTemp'; <span>//</span><span> 单引号不编译</span><span>echo</span> "<br>"<span>;
</span><span>echo</span> "在双引号里打印输出:"."<span>$sTemp</span>"; <span>//</span><span> 双引号编译</span>