How to plug in a variable for BCMath?

Hello everyone! I could really use some help with BCMath, which I have
just begun a project with. I require it's high-accuracy for a
calculator of mine, but I need to plug in a variable. Now, when I try this:
<?php
echo bcmul('1', '$var', 99); // 1;
?>
It doesn't work! This probably makes sense to you gurus out there, but
I don't know too much about PHP, I'm just starting. How the heck do you
plug in a variable in BCMath? One of you experts, PLEASE help me! The
deadline for the project is coming up soon... and I need the accuracy of
BCMath for a fact! Thanks, and please reply if you know!
.