This is a very interesting tutorial if you would like to build your own calculator written in PHP. It guides you through all the steps and it is very clean written and has a good error checking. It uses if, witch and other math functions.

And … from users recommendation I have inserted the php code used in the video:

<?php

if ( !$_POST['submit'] ) {
	echo "<table border=\"0\" cellspacing=\"3\" cellpadding=\"3\">\n";
	echo "<form method=\"post\" action=\"calc.php\">\n";
	echo "<tr><td><input type=\"text\" name=\"num1\"/></td>\n";
	echo "<td><select name=\"sign\"><option value=\"plus\">+</option>\n<option value=\"minus\">-</option>\n<option value=\"multi\">*</option>\n<option value=\"divide\">/</option></select></td>\n";
	echo "<td><input type=\"text\" name=\"num2\"/></td>\n";
	echo "<td><input type=\"submit\" name=\"submit\" value=\"Calculate\"/></td></tr>\n";
	echo "</form></table>\n"; 
} else {
	$num1 = $_POST['num1'];
	$num2 = $_POST['num2'];
	$sign = $_POST['sign'];
	
	$errors = array();
	
	$signs = array('plus', 'minus', 'multi', 'divide');
	
	if (!$num1) {
		$errors[] = "You did not supplied the first number";
	} else {
		if (!$num2) {
			$errors[] = "You did not supplied the second number";
		} else {
			if(!$sign) {
				$errors[] = "You did not supplied a sign to calculate your two numbers";				
			} else {
				if (!in_array($sign, $signs)) {
					$errors[] = "Invalid sign for calculation";
				} else {
					if($sign == "divide") {
						if ($num2 == 0) {
							$errors[] = "You cannot divide by 0. Please choose a different number to divide with";
						}
					}
					
					if (!is_numeric($num1)) {
						$errors[] = "Your first number is not numeric";
					} else {
						if (!is_numeric($num2)) {
							$errors[] = "Your second number is not numeric";
						}
					}					
				}
			}
		}
	}
	if (count($errors)>0) {
		echo "The fallowing errors occurred:<br>\n";
		foreach ($errors as $error) {
			echo $error."<br>\n";
		}
	} else {
		switch ($sign) {
			case 'plus':
				$equation = $num1 + $num2;
				$written = $num1 . " + " . $num2;
			break;
			
			case 'minus':
				$equation = $num1 - $num2;
				$written = $num1 . " - " . $num2;
			break;
			
			case 'multi':
				$equation = $num1 * $num2;
				$written = $num1 . " * " . $num2;
			break;
			
			case 'divide':
				$equation = $num1 / $num2;
				$written = $num1 . " / " . $num2;
			break;
			
			default :
				$equation = "";
				$written = "";
			break;
		}
		
		echo $written . " = " . $equation; 
	}
}
</pre>


    

Like in all programing languages, PHP too has math functions. The difference with other programing languages is that PHP is one of the best to recognize variables as numeric without previously define them as so. This is what you will learn in this www.thenewboston.com video tutorial.