Q:

Separate variable from string

I've got 2 vars - $j and $r
In string "$jx$r" php sees $jx as variable, but "x" is a string.

A:

You need to reformat your string a litte:
Tis might be easier to read:
echo $j."x".$r

But if you want it in one string:
echo "{$j}x$r";

See the manual page for double quoted strings, there is also an explaination how the curly braces work.

