PHP upload file can not be uploaded successfully, $_files[' screenshot ' [' Tmp_name '] is empty

Recently, in the 5th chapter of the book "Headfirst PHP & MySQL", "Using data stored in Files", a file upload application, an error occurred, that is, the file cannot be uploaded successfully. This problem has plagued me for a long time, but fortunately finally solved. The reason is that I uploaded a picture file larger than the value specified by the MAX_FILE_SIZE option in the HTML form 32768Bytes that 32KB caused the upload to succeed.

I used the XAMPP (Apache + MySQL + PHP + Perl) integration package and Zend Studio 10.6 as the PHP IDE development environment, in addition to PHP debugging I used the Xdebug, in Zend Studio10.6 Configuration I refer to the blog post Zend Studio 10.5 with XDebug debugging | Zend Debugger Description Drupal Source code (i)

The PHP code for my addscore.php is as follows:

<! DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 transitional//en" "Http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd ">
when I debug the above PHP code using Zend Sutdio10.6 I find "if (Move_uploaded_file ($_files[' screenshot ' [' Tmp_name '], $target)) {" The code block inside this line of code is not executed, and the value of the Super global variable $_files[' screenshot ' [' Tmp_name '] is empty, and then I print out the value of the $_files variable in JSON format before this line of code, as follows:
{"screenshot": {"name": "Penguins.jpg", "type": "", "Tmp_name": "", "Error": 2, "size": 0}}
As follows:

Then I query $_files[' screenshot ' [' Error '] for 2, surf the internet for a bit, about the $_files Super global variables are generally as follows:
Common $_files system function usages in the PHP programming language are:$_files[' myFile ' [' name '] Displays the original name of the client file. $_files[The MIME type of the ' myFile ' [' type '] file, such as "Image/gif". $_files[' myFile ' [' size '] the size of the uploaded file, in bytes. $_files[' myFile ' [' tmp_name '] The temporary file name stored, usually the system default. $_files[' myFile ' [' ERROR '] The file uploads the relevant error code. The following are the meanings represented by different codes:0. File upload is successful. 1; exceeds the size of the file size php.ini the system setting. 2. File size exceededthe value specified by the max_file_size option. 3; The file is only partially uploaded. 4; No files are uploaded. 5; Upload file size is 0. 
In addition, the query PHP reference manual about the Move_uploaded_file function is described below:

Example
Example #1 Uploading multiple files
<?php$uploads_dir = '/uploads '; foreach ($_files["Pictures"] ["error"] as $key = $error) {    if ($error = = Upload_ ERR_OK) {        $tmp _name = $_files["Pictures" ["Tmp_name"] [$key];        $name = $_files["Pictures" ["Name"] [$key];        Move_uploaded_file ($tmp _name, "$uploads _dir/$name");    
The reason is finally found, because I uploaded a 32768Bytes, or 32KB size of the penguins.jpg file caused the occurrence of $_files[' screenshot ' [' Error '] is 2 error, and $_files[' screenshot ' [' Tmp_name '] is empty, move_uploaded_file ($_files[' Screenshot '] [' tmp_name '], $target) function call returns FALSE,IF (Move_uploaded_file ($_files[' screenshot ' [' Tmp_name '], $target) ) {
...
The code block was not executed.