实际项目中，form表单的enctype属性设置成了“text/plain”，$_POST[]取值取不到。解决方法是删除该属性，或者设置为其他的另外两种属性值。遇到的问题，记录在此。
这里再来熟悉一下php中form表单中enctype属性的设置。
form表单中的enctype属性指定将数据发回到服务器时浏览器使用的编码类型。
下面是取值说明：
multipart/form-data： 窗体数据被编码为一条消息，页上的每个控件对应消息中的一个部分，不对字符编码。当使用有文件上传控件的表单时，该值是必需的。。
application/x-www-form-urlencoded： 窗体数据被编码为名称/值对。这是标准的编码格式。在发送前对所有字符进行编码（默认）。
text/plain： 窗体数据以纯文本形式进行编码，其中不含任何控件或格式字符，邮件处理中用到，其他地方很少用。将空格转换为 "+" 符号，但不编码特殊字符。抓包可见数据形式。
说明：
1）如果表单中有文件要上传，表单中form标签必须设置enctype="multipart/form-data"来确保匿名上传文件的MIME编码。默认情况下，表单的编码格式是 application/x-www-form-urlencoded，不能用于文件上传；
2）进行session处理时，如果将表单设置enctype="text/plain"，则通过$_POST,$_GET,$_REQUEST等是无法取到值的。这点一定要小心！！！
对于三种类型的进一步解释，实质参考如下：
enctype defines how the data should be formatted before sending. the two correct formats are application/x-www-form-urlencoded (which essentially sends things as key=valye&anotherkey=anothervalue, with http headers) and multipart/form-data (which splits each key up into a distinct section of data). text/plain means nothing should be done - its behaviour is essentially undefined between browsers, and was only ever used for automated email forms in the days before spam. use application/x-www-form-urlencoded for text forms (or leave this to use the default setting) and multipart/form-data for file attachment uploads
text/plain:
Content-Type: text/plain ================================================================================ foo=bar baz=The first line. The second line.
application/x-www-form-urlencoded:
Content-Type: application/x-www-form-urlencoded ================================================================================ foo=bar&baz=The+first+line.%0D%0AThe+second+line.%0D%0A
multipart/form-data:
Content-Type: multipart/form-data; boundary=---------------------------314911788813839 ================================================================================ -----------------------------314911788813839 Content-Disposition: form-data; name="foo" bar -----------------------------314911788813839 Content-Disposition: form-data; name="baz" The first line. The second line. -----------------------------314911788813839--
you can see how the first one is useful for passing values UNCHECKED directly into an email program (e.g. if your form uses a mailto: address, so it uses your visitor's email program to send) but is largely useless for programmatic access to fields. only the bottom 2 examples are valid for actual form-based form data