Encode string to match encoded form field name in PHP POST array

Question

The name of my HTML checkbox form field is something like this:

name = "Some File name.pdf"

The PHP POST array looks something like this (some characters replaced by underscore):

array(
 "Some_File_name_pdf" => "on"
)

Is there a PHP function I can use to convert a filename string exactly as it appears in the POST array? I am not interested in str_replace. I want to be able to do this:

$myfilename = $obj->getFileName(); // returns "Some File name.pdf"
$result = isset($_POST[some_encoding_function($myfilename)]);

The some_encoding_function should take a string like "Some File name.pdf" and return something like "Some_File_name_pdf";

Since both spaces and dots seem to be converted to underscores, there can't be a function that reliably converts them back. But then again, using non-URL-compatible characters in name attributes is probably a bad idea anyway. — Siguza
– Siguza, Commented Jun 15, 2015 at 6:58
I don't mean to convert back. But I mean convert again a filename into same format i.e. replace dots, spaces (and I don't know what else) to underscores. — Raouf Athar
– Raouf Athar, Commented Jun 15, 2015 at 7:15
The more practical solutions to this problem might be to a) not use arbitrary user supplied values as keys and/or b) encode such keys in a way that does not break after processing (i.e. encode it to something like Some_File_name_pdf yourself using your own method). — deceze
– deceze ♦, Commented Jun 15, 2015 at 9:39

Siguza · Accepted Answer · 2015-06-15 10:28:23Z

2

According to the PHP documentation:

Dots and spaces in variable names are converted to underscores.

This would require only a trivial string substitution.
However, according to a comment on that page:

The full list of field-name characters that PHP converts to _ (underscore) is the following (not just dot):
chr(32) ( ) (space)
chr(46) (.) (dot)
chr(91) ([) (open square bracket)
chr(128) - chr(159) (various)

If that comment is correct, then you should be good to go with a function like

function underscorify($s)
{
    return preg_replace('/[ \.\[\x80-\x9F]/', '_', $s);
}

Note however, that chr(128) - chr(159) is ambiguous, as it is not mentioned whether this is character-encoding-dependent or not.
It may refer to all ASCII characters from € to Ÿ, it may refer to all UTF-8 control characters from \u0080-\u009F, or it may simply be hardcoded to check the byte value for b >= 128 && b <= 159.

answered Jun 15, 2015 at 10:28

Siguza

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1 Comment

deceze Over a year ago

I'd put my money on it meaning any byte between 128-159. PHP is not encoding aware in its string handling and couldn't be for arbitrary requests, and the code chr(128) literally produces the byte x80.

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