$query = "UPDATE tidbokning SET " . $_POST['uppdatera'] . " = '$info_to_update' WHERE id = '1'";
or
$query = "UPDATE tidbokning SET {$_POST['uppdatera']} = '$info_to_update' WHERE id = '1'";
Generally speaking, the backticks are optional, unless the database, table, or column name is a reserved word in MySQL. (IMO best practise is never to use reserved words for identifiers.)
If you want variable interpolation in PHP, you must use double quotes. If you want to interpolate an array element, you must surround the variable name with curly brackets. Example:
<?php
$something = "word";
print "a $something"; # (double quotes) prints: a word
print 'a $something'; # (single quotes) prints: a $something
$test = array();
$test['hi'] = 'Hello';
print "{$test['hi']}, world!"; # prints: Hello, word!
print "{$test["hi"]}, world!"; # prints: Hello, word!
print "$test['hi'], world!"; # *syntax error*
print "$test["hi"], world!"; # *syntax error*
?>
Also, you should ALWAYS quote strings when used as associative array indexes (I missed this mistake before when looking at your code). See "Why is $foo[bar] Wrong?" in the PHP Manual:
http://php.net/types.array#language.types.array.foo-bar
Please spend some time with the PHP Manual sections on strings, arrays, and variable interpolation. Experiment until you're comfortable with how these work.
Jon Stephens
MySQL Documentation Team @ Oracle
MySQL Dev Zone
MySQL Server Documentation
Oracle