可以使用以下函数来检测指定表的指定字段是否存在：

```php
function isFieldExist($table, $field) {
$result = mysqli_query("SHOW COLUMNS FROM $table LIKE '$field'");
return mysqli_num_rows($result) > 0;
}
```

使用示例：

```php
if (isFieldExist("users", "username")) {
echo "The 'username' field exists in the 'users' table.";
} else {
echo "The 'username' field does not exist in the 'users' table.";
}
```