您编写:

$_imagePost = file_get_contents($_FILES['_imagePost']);

正确的语法是:

$_imagePost = file_get_contents($_FILES['_imagePost']['tmp_name']);

$_FILES是以下键的关联数组:

$_FILES is an associative array whit following keys:

• [名称] => 原始文件名；
• [type] =>上传文件的模仿类型；
• [tmp_name] =>临时文件路径(存储上传文件的位置)；
• [错误] =>错误；
• [大小] =>上传文件的大小.

• [name] => Original file name;
• [type] => mimetype of uploaded file;
• [tmp_name] => temporary filepath (where uploaded file is stored);
• [error] => error;
• [size] => size of uploaded file.

• 详细了解 $ _FILES