PHP: run function at certain places (preg_callback

Question

I'm trying to run a function whenever there's a [%xxx%] (acting as a placeholder, if you will), e.g.:

Bla bla bla blabla. Blablabla bla bla.
[%hooray%]
Blabla hey bla bla. [%yay%] blabla bla.

I'm pretty much a PHP beginner, but I managed to crack my head through and come up with the following (yay to me - I somehow managed to understand the basics of regular expressions!):

$maintext = preg_replace("#\[%(.{1,20})%\]#", "display_products('$1')"), $maintext);
echo $maintext; 

I tried working with the e modifier, I tried using preg_replace_callback, and it all does somewhat work - but I don't need the function to run before I echo the $maintext variable.

Any help on this guys?

edit The regex isn't the issue - that's working fine. I just need to figure out how to run the function only when I echo $maintext... preg_replace_callback runs the function immediately...

Answer 1

ob_start accepts a callback function, you can read more on this topic : Making all PHP file output pass through a "filter file" before being displayed

Answer 2

Because PHP is a single threaded procedural scripting language¹, it doesn't work in the way you want it to.

It is not possible to trigger the function to run only when you call echo², what you need instead is an extra variable.

$newVariable = preg_replace_callback("#\[%(.{1,20})%\]#", function($matches) {
  // Do your thang here
}, $maintext);

// $maintext remains the same as it was when you started, do stuff with it here

// When you come to output the data, do...
echo $newVariable;

There are other approaches to this problem, like wrapping the above code in a function that can be called on demand and using the output buffering callback (see footnote #2), but reading between the lines I think all of this would be overkill for what is a relatively simple problem.

For the record, I think your regex would be better if you narrow the allowed content down a it, it will potentially match characters you don't want it to. I suspect that this would be better:

#\[%(\w{1,20})%\]#

This will allow only the characters a-zA-Z0-9_ in the placeholder.

---

¹ Others may tell you PHP is now an object oriented language, but they are wrong. It still fundamentally works the same underneath and its still a scripting language, but now provides many OO-features.
² It is possible to do something very close to this, but it is very rarely ever a good idea and is far too advanced for dealing with this problem.

---

EDIT

It is important to note that when using either method to pass replacements through a callback (e modifier or preg_replace_callback()) the callback function should return the new string, rather than outputting it directly.

This is because the code in any statement is executed from the inside out, and the "inner" echo statement (in the callback function) will be executed before the "outer" echo statement (on the line where preg_replace() is called, so the order in which things are output will be incorrect.

For example:

$str = "This is my string which contains a %placeholder%";

echo preg_replace_callback('/%(\w+)%/', function($matches) {
  echo "replacement string";
}, $str);
// Wrong - outputs "replacement stringThis is my string which contains a "

echo preg_replace_callback('/%(\w+)%/', function($matches) {
  return "replacement string";
}, $str);
// Right - outputs "This is my string which contains a replacement string"

Answer 3

If you have a script that uses echo many times and you want the printed text modified, you can use output buffering:

ob_start();
/*
 * Lots of php code
*/

$maintext = ob_get_clean(); // now you capture the content in $maintext 

// than you modify the output
$maintext = preg_replace("#\[%(.{1,20})%\]#", "display_products($1)", $maintext);
echo $maintext;