How can PHP determine if a string is valid?

In PHP, you can use regular expressions to determine if a string is valid. Here is an example code:

function isValidString($str) {
   $pattern = '/^[a-zA-Z0-9_]+$/'; // 正则表达式，表示只包含字母、数字和下划线的字符串
   if (preg_match($pattern, $str)) {
      return true;
   } else {
      return false;
   }
}

// 使用示例
$str = "Hello_World123";
if (isValidString($str)) {
   echo "字符串合法";
} else {
   echo "字符串不合法";
}

In this example, a regular expression /^[a-zA-Z0-9_]+$/ is used to match strings that only contain letters, numbers, and underscores. The preg_match function is used to determine if the string matches the regular expression, returning true if it does and false if it does not.