How to loop through, match and replace?

Question

I have multiple strings with same curly braces I want to replace them as dynamic if I get the count as 1 then need to replace the first occurrence, If count as 2 then replaces t

Answer 1

There are a few issues with your code, you need to ensure that the variable in the preg_match_all() is the string your trying to search.

But the main problem is in the replacement part. You need to replace the current match value ($match) and replace it in a new string - currently you always replace the new match in the original string. Here I create $NewValue from the original string and keep replacing values in that...

if(preg_match_all("/\{\{[^{}]+\}\}/", $String, $matches)) {
    $NewValue = $String;
    foreach ($matches[0] as $match) {
        $Count++;
        $Query = "SELECT link FROM student WHERE linkVal = '".$match."'";
        $Result = $con->query($Query);

        if($row = $Result->fetch(PDO::FETCH_ASSOC)) {
            $NewValue = preg_replace("/".preg_quote($match)."/", 
                              $row["link"], $NewValue);
        }
    }

    echo json_encode($NewValue);

}

You should also look into using prepared statements as currently you may be open to SQL Injection problems.