在PHP IF语句中的多个条件中使用字符串

$mystring is declared as follows:

$mystring = "&& substr($losuj, 0, 1) == 'D' || substr($losuj, 0, 1) == 'K'";

I'd like to use $mystring (it's created by a loop earlier in my code) in the following statement:

while($row = mysql_fetch_array($wynik)) {
    $losuj = $row['wyraz'];
    if (!in_array($losuj, $losowe) 
    $mystring ){.......do something}
}

Is it even doable in PHP? I've tried echoing it normally, with {'$mystring'}, but I guess you can't interrupt loop statement just like that?

It's doable using eval.
See below a sample using a simpler condition

$losowe = array('Kool','Awesome');

$losuj = 'Kool';

$a = 'in_array($losuj,$losowe)';

$mystring = ' && substr($losuj, 0, 1) == "K";';

$condition = eval('return ' . $a . $mystring);

if ($condition == true){
    print 'done' . PHP_EOL;
}

Will output

done