Some main limitations of Variable Naming in PHP are given below:
1.In PHP their is not a fixed limit for Variable Name as compare to other programing language.
2.First letter of PHP Variable must start with a letters,numbers or underscore \"_\".
3.Variable Name should be a combination of Alpha-Numeric Characters or underscores(a-z,A-Z,0-9or_)
4.Variable name must be one word.If their is more than one word in our Variable Name.Than we can distinguish them with underscore(\"_\").Like $first_name.