PHP单引号和双引号对待变量的不同
如果一个变量放在单引号中，会被当作字符串来处理，如果是放在双引号中，则会被当值一个变量来处理（此时可以用 {}扩起来，也可以不用）。
<span style="color: #008080;">1</span> <span style="color: #000000;">php</span><span style="color: #008080;">2</span> <span style="color: #800080;">$txt</span> = "hello, this is from txt"<span style="color: #000000;">;</span><span style="color: #008080;">3</span> <span style="color: #0000ff;">echo</span> 'the word is {$txt}'; <span style="color: #008000;">//</span><span style="color: #008000;">the word is {$txt}</span><span style="color: #008080;">4</span> <span style="color: #0000ff;">echo</span> "the word is {<span style="color: #800080;">$txt</span>}"; <span style="color: #008000;">//</span><span style="color: #008000;">the word is hello, this is from txt</span>
