exec() didn't write file. (Returns '2' if try to write file with mysqldump)

Question

I'm trying to make a dump of a database with mysqldump called by an exec():

    $exec= exec('mysqldump --user=myUser --password=myPassword --host=myHost myDatabase > /pathto/www/myDumpfile.sql, $output, $return);

The myDumpfile.sql is not written, and var_dump shows:

$exec:   string(0) ""  
$output: array(0) { }  
$return: int(2)  

• The exec() is enabled on the system and runs well if the called program doesn't need to write a file.
• My code worked fine on Strato-server but after moving to 1blu I get this error.
• Writing some file to directory (/pathto/www/) works, if it is not done with exec() but with fwrite().
• I think I found out that there is a permissions issue. The trail leads to 'www-data', but I don't understand what that means. (But 'www-data' may be a false track.)

Can someone explain what 'www-data' is all about and/or how I can fix the error.
Thank you for helping!