PHP: Expressions

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Expressions

Aims:

• to see how to use interpolation in strings
• to see expressions involving numbers and strings

Writing strings using double quotes

• Using double quotes is more flexible
• There are more escape sequences when using double quotes:
  • to include double quotes, use \"
  • to include a backslash, use \\
  • to include a newline, use \n
  • some others: \$ \[ \] \{ \} ...
• Class exercise: What output will result from the following

echo "\"Marry me,\" he said.\n\"No way,\" she replied.";
echo "\"In that case, 1\\2 a pound of cheese, please," he said.";

Interpolation

• Suppose we have this:

  $surname = "Bond";
$firstname = "James";

• Painful:

  echo 'My name is ';
echo $surname;
echo ', ';
echo $firstname;
echo ' ';
echo $surname;


Interpolation continued

• Much nicer:

  echo "My name is {$surname}, {$firstname} {$surname}";

  Interpolation: in strings written using double quotes, variables in braces are replaced by their values
  (In fact the braces are optional most of the time)
• Not right:

  echo 'My name is {$surname}, {$firstname} {$surname}';

Arithmetic expressions

<?php
 $quantity = 1000;
 $pricePerBanana = 0.14;
 $discountPerCent = 25;

 $fullPrice = $quantity * $pricePerBanana;
 $discount = $fullPrice * $discountPerCent / 100;
 $actualPrice = $fullPrice - $discount;

 echo "<table>";
 echo "<tr><th>Full price:</th><td>{$fullPrice}</td></tr>";
 echo "<tr><th>Less discount:</th><td>{$discount}</td></tr>";
 echo "<tr><th>Actual price:</th><td>{$actualPrice}</td></tr>";
 echo "</table>";
?>

Arithmetic expressions

• The main arithmetic operators are +, -, *, /, %
• If there are multiple operators in an expression, in what order will they be evaluated?
  1. You can force an ordering using parentheses
  2. Precedence: *, /, % have higher precedence than +, -
  3. Associativity: +, -, *, /, % are left-associative
• Class exercises:
  1. 2 + 3 * 4
  2. 5 - 2 + 3
  3. (2 + 3) * 4
  4. 2 + ( 3 * 4)
  5. 18 / 3 * (3 - 1)

Arithmetic expressions

• These are the same:

  $price = $price + 5;
$price += 5;

  Similarly using -=, *=, /=, %=
• These are the same:

  $sectionNum = $sectionNum + 1;
$sectionNum += 1;
++$sectionNum;

  And $sectionNum++ is the same in many cases
• Similarly, with --$timeRemaining and $timeRemaining--

String expressions

• PHP has a string concatenation operator (.)

  $firstname = 'Hugh';
$surname = 'Jeego';
echo $firstname . $surname;
echo $firstname . ' ' . $surname;

• The operator will convert numbers to strings, if necessary
• You can use .=, similar to +=

String expressions

Class exercise: What is the output from the following?

$message = 'Ha';
echo $message . $message;
echo $message . $message . $message . 'Ho' . 'Ho' . 'Ho';
$message .= 'ppy';
echo $message . ' Xmas';
$age = 6;
echo $message . ' ' . $age . 'th Birthday';

PHP's 100+ built-in functions for strings

• trim($s) returns a copy of string $s with leading and trailing whitespace removed
• strtolower($s) returns a copy of $s changed to lowercase (strtoupper similarly)
• strpos($s1, $s2) returns the position of the first occurrence of string $s2 in string $s1 or false if not found
  • e.g. strpos("goodbye", "b") returns 4
• substr($s, $offset, $len) returns the substring of string $s that starts from position $offset and (optionally) is of length $len
  • e.g. substr("goodbye", 4, 2) returns "by"
  • e.g. substr("goodbye", 4) returns "bye"

Class exercise

• Suppose a PHP script stores a simple URL in variable $myURL
• You may assume that the URL stored in $myURL contains exactly three parts (scheme, hostname and pathname)
  • e.g. "http://www.cs.ucc.ie/cs1/index.html"
• Use strpos and substr to extract the following:
  • the scheme, i.e. everything up to and including "//"
  • the hostname, i.e. everything after "//" and up to but excluding the next "/"
  • the pathname, i.e. the rest
• Note! PHP has a built-in function (parse_url) for doing all this properly!