如何检查字符串是否是字母数字并且可以在PHP中包含空格？

I've been trying to make a secure search querying with PHP.

So far, this is my code, which is incorrect:

else if(!ctype_alpha($searchkey) && !is_numeric($searchkey) &&!ctype_print($searchkey)){
    $data['errMsg'] = "Please enter a valid search key.";
}

Although ctype_print is kinda good, it still accepts non-alpha characters, which is unsafe to SQL injections. What can I use that will allow whitespaces but disallow non-alphanumeric characters? Thank you. All answers will be greatly appreciated.

Edit:

So I got it. Sometimes, I get so silly. To keep from getting unsafe inputs, I used PHP's mysql_real_escape_string. Sorry guys.

use ctype_alnum() function:

!ctype_alnum($searchkey)

this checks if string is alpha-numeric

preg_match('/[a-zA-Z0-9\s]+/', $searchkey);

Try with:

function coolCheck($string) {
    return preg_match("/^[a-zA-Z0-9\s]*$/", $string);
}

If you're trying to defend against SQL Injection I would use a prepared statement (PDO example):

$stmt = $dbh->prepare('SELECT stuff FROM table WHERE input = ?');
$stmt->execute($searchkey);

foreach($stmt as $row){
    //do something
}

This is far more effective than any input sanitisation you can create.

If you're just looking to simply format the search query in a particular way please ignore this and use something like ctype_alnum($searchKey), PHP manual: http://php.net/manual/en/function.ctype-alnum.php