1. 判斷字符串是否是數字字符串

$str = "12345";
if (is_string($str) && is_numeric($str)) {
    echo "The string is a numeric string";
} else {
    echo "The string is not a numeric string";
}

2. 判斷字符串是否包含特定字符

$str = "Hello World";
$char = "W";
if (is_string($str) && strpos($str, $char) !== false) {
    echo "The string contains the character '$char'";
} else {
    echo "The string does not contain the character '$char'";
}

3. 判斷字符串是否以特定子字符串開頭

$str = "Hello World";
$subStr = "Hello";
if (is_string($str) && substr($str, 0, strlen($subStr)) === $subStr) {
    echo "The string starts with the substring '$subStr'";
} else {
    echo "The string does not start with the substring '$subStr'";
}

4. 判斷字符串是否以特定子字符串結尾

$str = "Hello World";
$subStr = "World";
if (is_string($str) && substr($str, -strlen($subStr)) === $subStr) {
    echo "The string ends with the substring '$subStr'";
} else {
    echo "The string does not end with the substring '$subStr'";
}

5. 判斷字符串是否全部為大寫或小寫

$str = "hello world";
if (is_string($str) && ctype_lower($str)) {
    echo "The string is all lowercase";
} else if (is_string($str) && ctype_upper($str)) {
    echo "The string is all uppercase";
} else {
    echo "The string is mixed case";
}

這些是php is_string函數的一些高級應用，可以用來更詳細地判斷字符串的特性和內容。