In order to upgrade the script to PHP8x, you need to fix 2 locations:
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[1] web/lib/sys/CPreProc.class.php
------------------------------------------------------------------------
/*
	public static function reverseMagicQuote() {
		$func = "reverseMagicQuote_stripslashes_deep";
		if ( !function_exists( $func ) && get_magic_quotes_gpc() ) {
			define( 'REVERSE_MAGIC_QUOTE', $func );
			function reverseMagicQuote_stripslashes_deep( $value ) {
				$value = is_array( $value ) ?
					array_map( REVERSE_MAGIC_QUOTE, $value ) :
					stripslashes( $value );
				return $value;
			}
			$_POST = array_map( $func, $_POST );
			$_GET = array_map( $func, $_GET );
			$_COOKIE = array_map( $func, $_COOKIE );
			$_REQUEST = array_map( $func, $_REQUEST );
		}
	}
*/
	public static function reverseMagicQuote(){
		$ver_req="5.4";
		$ver_cur=phpversion();
		if(strnatcmp($ver_cur,$ver_req)>=0){
			// equal or newer 
		}else{
			$func="reverseMagicQuote_stripslashes_deep";
			if(!function_exists($func)&&
				function_exists("get_magic_quotes_gpc")&&
				get_magic_quotes_gpc()){
				define('REVERSE_MAGIC_QUOTE',$func);
				function reverseMagicQuote_stripslashes_deep($value){
					$value=is_array($value)?
						array_map(REVERSE_MAGIC_QUOTE,$value):
						stripslashes($value);
					return $value;
				}
				$_POST=array_map($func,$_POST);
				$_GET=array_map($func,$_GET);
				$_COOKIE=array_map($func,$_COOKIE);
				$_REQUEST=array_map($func,$_REQUEST);
			}
		}
	}
========================================================================
[2] web/lib/app/CTpl.class.php : line 54
------------------------------------------------------------------------
/*
	public static function tag( $vikey = null, $param = null, $tag ) {
*/
	public static function tag( $vikey = null, $param = null, $tag = "tr" ) {
========================================================================

========================================================================
[3] web/codegen/mod/btpl/CMPage.class.php : line 74
------------------------------------------------------------------------
Before
		CApp::set( "page-title", $this->item['title'] );
------------------------------------------------------------------------
After
		if(isset($this->item['title'])){
			CApp::set( "page-title", $this->item['title'] );
		}
========================================================================