When the value passed by Ajax contains "+"

Today, when I use ajax to pass a value, I encountered some problems when the passed value contains +, such as 'a + B ', the requested page 'a + B 'is changed to 'a B', and the value to be passed is processed using regular expressions.

Var key = Document. getelementbyid ("key"). value;
Key = key. Replace (/\ +/g, "% 2B"); // G.

Ajax. Send ("Key =" + key );

In this way, the '+' is retained in the value received by the requested page.

Although it has been solved, I am puzzled that there will be no errors in passing post values in PHP forms, and why do I have to escape it when Ajax is used?