fopen() will open a resource in the same directory as the file executing the command. In other words, if you're just running the file ~/test.php, your script will create ~/myText.txt.
This can get a little confusing if you're using any URL rewriting (such as in an MVC framework) as it will likely create the new file in whatever the directory contains the root index.php file.
Also, you must have correct permissions set and may want to test before writing to the file. The following would help you debug:
$fp = fopen("myText.txt","wb");
if( $fp == false ){
//do debugging or logging here
}else{
fwrite($fp,$content);
fclose($fp);
}