Addslashes is *never* the right answer, it's (ab)use can lead to security exploits!

if you need to escape HTML, it's (unfortunately)
<?php
echo htmlentities($html, ENT_QUOTES|ENT_SUBSTITUTE|ENT_DISALLOWED);
?>
if you need to quote shell arguments, it's
<?php
$cmd.= " --file=" . escapeshellarg($arg);
?>
if you need to quote SQL strings it's
<?php
$sql.= "WHERE col = '".$mysqli->real_escape_string($str)."'";
?>
or
<?php
$sql.= "WHERE col = " . $pdo->quote($str);
?>
if you need to quote javascript/json strings its
<?php
let str = <?=json_encode($str, JSON_THROW_ON_ERROR);?>;
?>

if you need to quote a string in xpath it's
<?php
//based on https://stackoverflow.com/a/1352556/1067003
function xpath_quote(string $value):string{
    if(false===strpos($value,'"')){
        return '"'.$value.'"';
    }
    if(false===strpos($value,'\'')){
        return '\''.$value.'\'';
    }
    // if the value contains both single and double quotes, construct an
    // expression that concatenates all non-double-quote substrings with
    // the quotes, e.g.:
    //
    //    concat("'foo'", '"', "bar")
    $sb='concat(';
    $substrings=explode('"',$value);
    for($i=0;$i<count($substrings);++$i){
        $needComma=($i>0);
        if($substrings[$i]!==''){
            if($i>0){
                $sb.=', ';
            }
            $sb.='"'.$substrings[$i].'"';
            $needComma=true;
        }
        if($i < (count($substrings) -1)){
            if($needComma){
                $sb.=', ';
            }
            $sb.="'\"'";
        }
    }
    $sb.=')';
    return $sb;
}
$xp->query('/catalog/items/item[title='.xpath_quote($var).']');
?>
if you need to quote strings in CSS its
<?php
// CSS escape code ripped from Zend Framework ( https://github.com/zendframework/zf2/blob/master/library/Zend/Escaper/Escaper.php )
function css_escape_string($string)
{
    $cssMatcher = function ($matches) {
        $chr = $matches[0];
        if (strlen($chr) == 1) {
            $ord = ord($chr);
        } else {
            $chr = mb_convert_encoding($chr, 'UTF-16BE', 'UTF-8'); // $this->convertEncoding($chr, 'UTF-16BE', 'UTF-8');
            $ord = hexdec(bin2hex($chr));
        }
        return sprintf('\\%X ', $ord);
    };
    $originalEncoding = mb_detect_encoding($string);
    if ($originalEncoding === false) {
        $originalEncoding = 'UTF-8';
    }
    ;
    $string = mb_convert_encoding($string, 'UTF-8', $originalEncoding); // $this->toUtf8($string);
                                                                        // throw new Exception('mb_convert_encoding(\''.$string.'\',\'UTF-8\',\''.$originalEncoding.'\');');
    if ($string === '' || ctype_digit($string)) {
        return $string;
    }
    $result = preg_replace_callback('/[^a-z0-9]/iSu', /*$this->*/$cssMatcher, $string);
    // var_dump($result);
    return mb_convert_encoding($result, $originalEncoding, 'UTF-8'); // $this->fromUtf8($result);
}

?>

- but never addslashes.