substr function weirdness !!!!

flanagaj

I am making the following function call

substr($courseCodes, 0, 3);

and it prints out SC

which seems strange as when I do

PRINT $courseCodes;
PRINT substr($courseCodes, 0, 3);

I get

SCN103,SCN123
SC

WHAT IS GOING ON HERE. I have even tried

substr(trim($courseCodes), 0, 3);

Am I thick or is something strange.

Thanks

Juckky

flanagaj

As the var is assigned from a blob data type it seems to be allocating 2bytes per character.

nemonoman

my guess is that the string starts with a non printing character (ex 00). Or the length of the string might be stored in the first character, for example.

Try this
$firstchar=substr($courseCodes,0,1);
echo "The first character of \$coursecodes is $firstchar.<BR>";
$charord=ord($firstchar);
echo "The ascii value of the first character of \$courseCodes is $charord.<BR>";

flanagaj

I checked the type of the var and it is a string, but it seems to be assigning 2 bytes per char.

ie. if I echo the var I get 'SCN'

if I then do

if($var == 'SCN')

it evaluates to false.

if I then do

$newVar = $var[0].$var[2].$var[4];

and then do

if($var == 'SCN')

it evaluates to true;

nemonoman

Please run my code and post result

if I then do

$newVar = $var[0].$var[2].$var[4];

and then do

if($var == 'SCN')

it evaluates to true;

I think you've included some typos in this snippet. If you haven't included typos, I am very confused by the results.

flanagaj

There are no typos in the code.

Basically, the var that is causing me a problem is assigned from a query on a MySQL dbase where the column type is = BLOB. This is what I think is causing the problem.

Did you run the code I posted ?

nemonoman

I respect you, but here's what:
I don't have any idea what you've got in your database: you do. You've got to run code to figure things out, not me.

Next: I seriously doubt that this code does what you say. Maybe I'm wrong, but let me work through my 'there's a typo' logic:

if($var == 'SCN') \ you say this returns false.

$newVar = $var[0].$var[2].$var[4];

\suggests that you are taking every other character, of $var, yes?

if($var == 'SCN')

evaluates as true

\ don't you mean if ($newVar=='SCN') evaluates as true?

if, after the act of creating $newVar, $var has NOW CHANGED to =='SCN', then I am very bewildeered.

However, if in fact $newVar=='SCN', then you're on the right track to analyzing your problem. MySQL has messed around with the string to store it in BLOB form, probably by padding the stored data with empty bytes.

WHY are you storing a string as a blob in the first place? Storing a string as varchar or text seems much simpler and more to the point. I can't think of many reasons why storing a string as a blob is to your advantage, particularly if you are trying to extract data from the blob.

flanagaj

Apologies !!!!

You are right I meant if ($newVar=='SCN').

The reason I am using a BLOB is that the field maybe used for a whole page of HTML ie Course Overview or it can be used for course codes. I dont know until runtime. + rather than write the admin screens to the web site www.scnltd.com I have written a number of Access forms for the team to use. And access only supports text upto 255 chars and therefore BLOB was the only option.

What is strange is that when I view the HTMl source it reads

S C N 1 0 3

There seems to be a spare char in between everything. I can get around it. I dont think I will ever solve the issue.

Cheers

Juccky

flanagaj

I will try utf8_decode($var) to see if that gets rid of the empty bytes.

I will let you know. Gonna crash out.

flanagaj

utf8_decode failed

nemonoman

here:

accessblobtostr($blob){
$len=strlen($blob);
$count=0;
$str='';
while($count<$len){
$str.=$blob[$count];
$count.=$count+2;
}
return $str;
}

flanagaj

Thanks for the code snippet. I was going to resort to extracting every other byte of the string. Seems a hacky way to do it and would love to know what is going on under the covers and why I have double byte chars.

Thanks for your help !!!

drawmack

according to the mysql manual (pg #462, pdf pg 491) a blod is the same thing a varchar binary.

From the mysql manual:

The BINARY attribute
means that column values are sorted and compared in case-sensitive fashion according to
the ASCII order of the machine where the MySQL server is running. BINARY doesn’t a®ect
how the column is stored or retrieved.

this doesn't seem to be any help, or it is. You are storing into a blob column so there is not conversion or anything happening let's look at character encoding for a second.

In unicode the basic latin upper case alphabet occupies the following namespace:
(hex) 0041 - 005A
and the lower case alphabet occupies the following namespace:
(hex) 0061 - 007A
(http://www.unicode.org/charts/)

In ascii the basic latin upper case alphabet occupies the following namespace:
(hex) 41 - 5A
and the lower case alphabet occupies the following namespace:
(hex) 61 - 7A
(http://www.asciitable.com/)

What this means is that the string 'abcd' in ascii is 61626364 in unicode it is 0061006200630064.

If I have a unicode string and I attempt to read it as ascii with a function like substring and take the first four characters the function does this
00 61 00 62. Since 0 is a non-printable ascii character my output is just 61 62 or 'ab'.

you might try 1 of two things:
this function http://us2.php.net/utf8_decode

or this function:

<?php
function utf16_to_ascii($utf_string) {
  for($i=0;$i<strlen($utf_string);$i++) {
    $temp = substr($utf_string,$i,1);
    if(ord($temp) != 0) {$retVal .= $temp;}
  } //end for
  return $retVal;
} //end utf16_to_ascii
?>

nemonoman

This BLOB problem made no sense until flanagaj explained that he was using access not MySQL.

drawmack

I missed that, in access blobs are unicode which is what is explained in my post.

Weedpacket

Using Access in this context is a mistake in itself.

drawmack

access is the best database ever created, except for mssql server.

looks nervously over shoulder for ms police

timj

Can an error like this (caused by 2 bytes chars) also occur when you read a file?

I know I'm absolutely new to Php (and MySQL), but can someone explain what you mean with a "blob" data type? I've never heard it before.

Don't worry about ms police, they are too busy stopping worms 🙂