php FILE POST upload without save

When the form is submitted, check the $_FILES['input_name']['tmp_name'] property. This is the path to your file saved in a temporary /tmp system path. You can proceed reading the name using, say, file_get_contents() and then simply forget the file. System will take care of removing it.

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Just to stand out of the other answers, you could theoretically read the file without even uploading it using JavaScript, see http://www.html5rocks.com/en/tutorials/file/dndfiles/. Then submit only the data you need as part of AJAX request.

If its text file (assuming size is not that huge) usually you could read it by

$contents= file_get_contents($_FILES['file']['tmp_name']);

If you are not sure which type of upload it was, check the request method

if(strtolower($_SERVER['REQUEST_METHOD'])=='post')
    $contents= file_get_contents($_FILES['file']['tmp_name']);
elseif(strtolower($_SERVER['REQUEST_METHOD'])=='put')
    $contents= file_get_contents("php://input");
else
    $contents="";