我是否允许将单引号，双引号heredoc语法或nowdoc语法字符串直接放入使用字符串的函数参数中？

Am I allowed to put single quoted, double quoted heredoc syntax or nowdoc syntax strings directly into functions whose parameters require a string like for example strlen('string text') or strlen("some more string text") instead of including a variable for example strlen($str);?

If not why?

Yes. You can. You do not need to store it in a variable

Yes, you can use any syntax for creating strings.

Note, however, that you have to be careful when using heredoc/nowdoc syntax with function calls: the final line of the string can't contain anything except the identifier:

var_dump(<<<HERE
foo
HERE
);

You are allowed to do that unless function expects string variable to be passed by reference:

// '&' means that argument is passed by reference
function requestStringAsVariable(&$str) {
    $str = '*' . $str . '*';
}

$str = 'test';
requestStringAsVariable($str);
echo $str; // outputs '*test*';

requestStringAsVariable('foo'); // won't work, as function expects variable