PHP - replace text and check for file?

This is my proposed solution using preg_match_all instead of preg_replace or preg_replace_callback. I have made a couple of assumptions: 1) The images are always digits followed by dot jpg, and 2) There may be other text in between the images. I'm sure you can adjust those as required.

//sample text to search and replace on
$text = 'Some other text \<img>30.jpg\<img> something in between \<img>50.jpg\<img> something else \<img>40.jpg\<img>';

//initialize an array to hold the matches
$matches = [];

//this is here to mock up the function you said you needed to call
//this code simply fetches the file contents
function image_text_print($filename)
{
  return file_get_contents($filename);
}

//find all matches of \<img>30.jpg\<img> where 30 can be any number made up of 1 or more digits (adjust the regex to your requirements)
if(preg_match_all('#\\\<img>(\d+).jpg\\\<img>#', $text,$matches))
{
  //preg_match_all gives us 2 arrays, one with the matched text and one with the image numbers
  foreach($matches[0] as $matchInd=>$matchText)
  {
    //build up the replacement text
    $replacement = '<img scr="img/'.$matches[1][$matchInd].'.jpg">';
    
    //create the filename and then check if it exists
    $filename = 'img/'.$matches[1][$matchInd].'.txt';
    if(is_file($filename))
    {
      //if the file exists call your function and append results
      $replacement .= image_text_print($filename);
    }

    //finally replace the matched text
    $text = str_replace($matchText,$replacement,$text);
  }
}