PHP - How to avoid replacing a replacement string - php

Question

I'm working on a script that allows students to input their answers into a form and gives them instant feedback on their answers.
I start with a string ($content) that contains the complete task with the gaps in square brackets, something like this:
There's [somebody] in the room. There isn't [anybody] in the room.
Is [anybody] in the room?
Now the script recognizes the solutions (somebody, anybody, anybody) and saves them in an array. The student's answers are also in an array.
To see if the answer is correct, the script checks if $input[$i] and $solution[$i] are identical.
Now here is the problem: I want the script to replace the placeholders with an input box where the solution is wrong and the solution in green where it's correct. This updated version of $content is then shown on the next page.
But if there are two identical solutions, this results in multiple replacements as the replacement is replaced again...
I tried preg_replace with a limit of 1 but this doesn't do the trick either as it doesn't skip solutions that have already been replaced.
$i=0;
while ($solution[$i]){
//answer correct
if($solution[$i] == $input[$i]){
//replace placeholder > green solution
$content = str_replace($solution[$i], $solution_green[$i], $content);
}
//answer wrong
else{
//replace placeholder > input box to try again
$content = str_replace($solution[$i], $solution_box[$i], $content);
}
$i++;
}
print $content; //Output new form based on student's answer
Is there any way to avoid replacing replacements?
I hope I didn't ramble too much... Have been wracking my brain for ages over this problem and would appreciate any suggestions.

Answer 1

The way I've approached this is to split the original content into segments which relate to the markers in the text. So then you explode() the original text by ], you end up with...
Array
(
[0] => There's [somebody
[1] => in the room. There isn't [anybody
[2] => in the room.
Is [anybody
[3] => in the room?
)
As you can see, each array element now corresponds with the answer/solution numbers. So when replacing the text, it changes $parts[$i] instead. Also as a safeguard, it replaces [text to make sure, there are other solutions, but this should do the job.
At the end, the code then rebuilds the original content by using implode() and using ] to add it back.
$parts = explode("]", $content);
$i=0;
while (isset($solution[$i])){
//answer correct
if($solution[$i] == $input[$i]){
//replace placeholder > green solution
$parts[$i] = str_replace("[".$solution[$i], "[".$solution_green[$i], $parts[$i]);
}
//answer wrong
else{
//replace placeholder > input box to try again
$parts[$i] = str_replace("[".$solution[$i], "[".$solution_box[$i], $parts[$i]);
}
$i++;
}
$content = implode( "]", $parts);

Answer 2

You may use sprintf()/vsrpintf() function to replace positional placeholders, but first you'd have to prepare sentence pattern for it. Each "solution placeholder" should be replaced with %s, so that later sprintf() could replace each one with corresponding string.
You may do that within loop:
$fields = [];
while (isset($solution[$i])) {
$fields[] = ($solution[$i] === $input[$i])
? $solution_green[$i]
: $solution_box[$i];
//doesn't matter if you replace more than one here
$content = str_replace($solution[$i], '%s', $content);
$i++;
}
print vsprintf($content, $fields);
//or for php>=5.6: sprintf($content, ...$fields);
This is an easy-fix solution to current state of your code. It may be refactored (pattern replacement while parsing correct words, green/box arrays may be replaced with methods producing string you need... etc.)