Question

Here’s something interesting I’ve just found out regarding str_replace().
In my function I need to make two mysql queries which are the same with the slight change on sorting the result. As an argument I get string containing an order clause, for example ‘surname ASC’ but it could be ‘surname DESC’ as well. Now I wanted to easily switch to the reverse sorting using str_replace and I thought this should do the trick:

str_replace(array('ASC', 'DESC'), array('DESC', 'ASC'), $subject)

In my mind it should change all occurences of ASC with DESC and all occurences of DESC with ASC. Since the string contains only one of the two I should get reversed order clause.
However this is not the case. The output of the above code is the same string.

I did some testing and it tourned out that these calls do what you want them to do:

str_replace('ASC', 'DESC', $subject)
str_replace(array('ASC'), array('DESC'), $subject)

In my opinion this is weird because

array('ASC', 'DESC') != array('DESC', 'ASC')

Why then PHP would consider this as equal? Is there any other way to make this kind of replacement easily?

Answer 1

In this scenario, you can use strtr like so:

strtr($subject, array('ASC' => 'DESC', 'DESC' => 'ASC'));

This will work because

If given two arguments, the second should be an array in the form
array('from' => 'to', ...). The return value is a string where all the
occurrences of the array keys have been replaced by the corresponding
values. The longest keys will be tried first. Once a substring has
been replaced, its new value will not be searched again.

See it in action.

With str_replace what happens, as you have already found out, is that first ASC is replaced with DESC and then DESC is replaced back to ASC, for a grand total of nothing done.