str_replace() not displaying video when inserted after an image - php

Question

I have a variable as follows:
$post= 'TO UPLOAD DO THIS <img src="Christmas.PNG"> and this <iframe src="https://www.youtube.com/embed/aAkMkVFwAoo"></iframe>';
With the <img> tag I want to process the image to display diferently. I also want the video to be processed differently (customisable). I have done the following:
//Image processing:
preg_match_all('/<img src=\"(.*?)\">/',$post,$matches);
if(sizeof($matches[1])>0){
$toRmove=[];
$toAdd = [];
foreach($matches[1] as $m){
$value = $m;
$toRemove[]='<img src="'.$value.'">';
$toAdd[]='<img src="images/'.$value.'" width=20% height=20%>';
}
$new_message = str_replace($toRemove,$toAdd,$post);
$post= $new_message;
}
//video processing
preg_match_all('/<iframe src=\"(.*?)\">/',$post,$matches);
if(sizeof($matches[1])>0){
$toRmove=[];
$toAdd = [];
foreach($matches[1] as $m){
$value = $m;
$toRemove[] ='<iframe src="'.$value.'">';
$toAdd[] ='<iframe width="560" height="315" src="https://www.youtube.com/embed/aAkMkVFwAoo" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen>';
}
$new_message = str_replace($toRemove,$toAdd,$post);
$post= $new_message;
}
echo $post;
When I echo $toAdd[0], the video displays correctly with what i want. However, after the str_replace($toRemove,$toAdd,$post); and then i echo $post the video just disappears but the image is still there and displaying correctly.
So the video processing is working but when it i go to add it, the video doesn't appear just blank.
Why is this occurring? Thanks.