php在数学函数中提取数字和变量[关闭]

I have a math function, for example:

string = "3x+6.5y-23z"

I need to extract the function and get x, y, z; give value x = 6, y = 7, z = 8; and solve.

you can do it like this

<?php 
  $x = 6;
  $y = 7;
  $z = 8;

 echo  $string = 3*$x+6.5*$y-23*$z;
  ?>