您需要为’和”指定替换字符串，因为’ != ‘和” != “

var strToReturn = "“Hello” ‘world’"
strToReturn = strToReturn.replacingOccurrences(of: "&", with: "&")
strToReturn = strToReturn.replacingOccurrences(of: "<", with: "&lt;")
strToReturn = strToReturn.replacingOccurrences(of: ">", with: "&gt;")
strToReturn = strToReturn.replacingOccurrences(of: "‘", with: "&apos;")
strToReturn = strToReturn.replacingOccurrences(of: "“", with: "&quot;")
strToReturn = strToReturn.replacingOccurrences(of: "’", with: "&apos;")
strToReturn = strToReturn.replacingOccurrences(of: "”", with: "&quot;")

这是因为”不同于“，’不同于‘。所以你也需要添加这些行。

strToReturn = strToReturn.replacingOccurrences(of: "’", with: "'")
strToReturn = strToReturn.replacingOccurrences(of: "”", with: """)

这会给予你预期的结果

如果你打印字符串的ascii值，你会看到引号不是同一个unicode字符。因此，请确保使用相同的unicode字符或处理两种情况

strToReturn.characters.map{print($0, "\(String($0.unicodeScalars.first!.value, radix: 16, uppercase: true))")}

“ 201C
H 48
e 65
l 6C
l 6C
o 6F
” 201D
  20
‘ 2018
w 77
o 6F
r 72
l 6C
d 64
’ 2019

你的代码和我一起工作得很好。我只是改变了strings，正如我在评论中提到的：
如果有人想知道字符串中的单引号和双引号是如何生成的---按住alt/option并按下方括号/方括号键
只要改变字母使用组合键，它将工作

下面扩展非常有用

var toReplaceSmartQuotes: String {
        return self.replacingOccurrences(of: "‘", with: "'").replacingOccurrences(of: "’", with: "'")
            .replaceCharacters(characters: "”", toSeparator: #"""#)
    }
    
    func replaceCharacters(characters: String, toSeparator: String) -> String {
        let characterSet = CharacterSet(charactersIn: characters)
        let components = components(separatedBy: characterSet)
        let result = components.joined(separator: toSeparator)
        return result
    }