Cookie setting failed

Why does the cookie settings fail? I have a program like this: if (setcookie ('me', 'mycookie ') {echo & nbsp; "cookie set";} echo & nbsp; "logged on "; the running result is "logged on", but "cookie set" is not displayed. This indicates why the cookie setting fails?
I have such a program:
If (setcookie ('me', 'mycooker '))
{
Echo "cookie set ";
}
Echo "logged on ";

The running result is "logged on", but "cookie set" is not displayed. This indicates that the cookie settings failed.

Why can't I set this program? Is there a syntax error? What are the possible causes of failure? (My browser supports cookies:
------ Solution --------------------
Setcookie () defines a cookie sent together with other HTTP headers. Like other headers, cookies must be sent before any other output of the script (this is a protocol restriction ). This requires that the call of this function be placed before any output, including, labels, and any spaces. If any output exists before setcookie () is called, this function fails and returns FALSE. If the setcookie () function runs successfully, TRUE is returned.