Answer:
b. & Puihaha
Step-by-step explanation:
PHP variables are supposed to start with a $ sign.
Other rules followed by PHP variables are:
Starting character in variable name (after $) must be letter or _.
Other characters in variable names can be letters,numbers or _.
a. $_Puihaha
c. $Pui_haha
d. $Puihaha
satisfy these rules.
Whereas , b. & Puihaha does not satisfy as it starts with '&'. Moreover it also contains an invalid space character in the variable.