In order to find if a variable holds an actual number or a string containing characters that can be translated into a number you can use:

<?php
"Hello World"
?>

A. Error B. Hello World C. Nothing D. Missing semicolon error

<?php
print_r "Hello world"
?>

A. Error B. Hello World C. Nothing D. Missing semicolon error

<?php
$color = "red";
echo "$color";
echo "$COLOR";
echo "$Color";
?>

A. redredred B. redred C. red D. Error

<?php
 # echo "Hello world";
 echo "# Hello world"; 
?>

A. # Hello world B. Hello world# Hello world C. Hello world D. Error