How do I include JavaScript code within HTML that is inside a PHP string?

Solution:

Your example is basically correct, but it needs a couple of adjustments:

Change the greedy selector .* to a non-greedy version .*? so it won’t overmatch.

Fix the generated JavaScript. The $1 placeholder is inserted literally, which makes the JavaScript invalid. You’ll need to escape it properly and pass it as a string.

<?php
$record = array("DESCRIPTION" => '<iframe src="http://www.google.com"></iframe>');

$replacement  = '$1<br>';
$replacement .= '<button type="button" onclick="download()">تحميل</button>';
$replacement .= '<script>';
$replacement .= 'function download() {';
$replacement .= '   APP.CallSub("download", true, YouTubeGetID(\'$1\'));';
$replacement .= '}';
$replacement .= 'function YouTubeGetID(url){';
$replacement .= '   var ID = "";';
$replacement .= '   url = url.replace(/(>|<)/gi,"").split(/(vi\/|v=|\/v\/|youtu\.be\/|\/embed\/)/);';
$replacement .= '   if(url[2] !== undefined) {';
$replacement .= '       ID = url[2].split(/[^0-9a-z_\-]/i);';
$replacement .= '       ID = ID[0];';
$replacement .= '   } else {';
$replacement .= '       ID = url;';
$replacement .= '   }';
$replacement .= '   return ID;';
$replacement .= '}';
$replacement .= '</script>';

$record["DESCRIPTION"] = preg_replace(
    "/(<iframe.*?<\/iframe>)/U",
    $replacement,
    $record["DESCRIPTION"]
);

echo $record["DESCRIPTION"];
?>

Output

<iframe src="http://www.google.com"></iframe><br>
<button type="button" onclick="download()">تحميل</button>
<script>
function download() { 
    APP.CallSub("download", true, YouTubeGetID('<iframe src="http://www.google.com"></iframe>'));
}
function YouTubeGetID(url){
    var ID = "";
    url = url.replace(/(>|<)/gi,"").split(/(vi\/|v=|\/v\/|youtu\.be\/|\/embed\/)/);
    if(url[2] !== undefined) {
        ID = url[2].split(/[^0-9a-z_\-]/i);
        ID = ID[0];
    } else {
        ID = url;
    }
    return ID;
}
</script>