Convert file into byte array

Try with explode() method like this:

$content = file_get_contents($file['uri']);
$arr=explode(" ",$content);
foreach ($arr as $value) {
    print_r(unpack("N*",$value));
}

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What do you want to do with the file data?
When you read the file in using file_get_contents you get a string which can be accessed like this

$content = file_get_contents($file['uri']);
for($i = 0; $i < strlen($content); $i++) {
  echo $content[$i] . "<br>";
}

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I think if you look at the PHP man page http://php.net/manual/en/function.unpack.php , you will see that 'unpack' is not what you want to do.  All files that are not otherwise processed are 'byte arrays' that can be accessed like Julian shows above with a numerical index.  'Real' arrays in PHP have two parts, the Key and the Value.  That will make your array at least twice the size it needs to be.

Like Julian asked, what do you want to do with it?