PHP: Strings

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Strings

Aims:

to learn what happens when you don't initialise
to learn about operators and expressions involving strings
to learn how to use interpolation in strings

Initialisation

What happens if you fail to initialise a variable?
- The variable contains a special value of a special type, NULL
What happens if you use a variable before initialising it?
- In most PHP installations, NULL is silently converted to the empty string ('') (or 0, 0.0 or false as appropriate)
- In our PHP installation, NULL is converted to the empty string (or 0, 0.0 or false as appropriate)
- But we have configured our PHP so that it also displays a warning message at runtime
Advice: always initialise your variables!

Class exercise I: what happens?

<body>

    <?php
        $surname = $_GET['surname'];
        
        echo '<p>';
        echo 'Hello ';
        echo $firstname; 
        echo ' ';
        echo $surname;
        echo '</p>';
    ?>
 
</body>

Class exercise II: what happens?

<body>

    <?php
        $firstname = $_GET['firstname'];
        $surname = $_GET['surname'];
        
        echo '<p>';
        echo 'Hello ';
        echo $frstname; 
        echo ' ';
        echo $surname;
        echo '</p>';
    ?>
 
</body>

Class exercise III: what happens?

<body>

    <?php
        $firstname = $_GET['firstname'];
        $surname = $_GET['surname'];
        
        echo '<p>';
        echo 'Hello ';
        echo $firstName; 
        echo ' ';
        echo $surname;
        echo '</p>';
    ?>
 
</body>

Class exercise IV: what happens?

<body>

    <?php
        $firstname = $_GET['firstname'];
        $surname = $_GET['surname'];
        
        echo '<p>';
        echo 'Hello ';
        echo firstname; 
        echo ' ';
        echo $surname;
        echo '</p>';
    ?>
 
</body>

Expressions, operators and operands

An expression in PHP is a bit of PHP that can be evaluated to compute a value
E.g.

2 + 3
2 + (3 * 4)
An expression contains operators, e.g. +, *
An operator does something to some values, its operands, to compute a value

String expressions

PHP has a string concatenation operator (.)
E.g.
'Hello ' . 'world!' evaluates to 'Hello world!'

Class exercise: What is the output from each of the following?

$word1 = 'Happy';
$word2 = 'Christmas';
$laugh = 'Ho';

echo $word1 . $word2;
echo $word1 . ' ' . $word2;
echo $laugh . $laugh . $laugh;

$wishes = $word1 . ' ' . $word2;
$laugh = $laugh . ' ' . $laugh . ' ' . $laugh;

echo $wishes;
echo $laugh;
echo 'Ho ' . $laugh . ' Ho';

What do the variables contain at the end of all this?

Using string concatenation in programs

<body>

    <?php
        $firstname = $_GET['firstname'];
        $surname = $_GET['surname'];
        
        echo '<p>Hello ' . $firstname . ' ' . $surname . '</p>';
    ?>
 
</body>

Single quotes and double quotes

We've been writing strings in single quotes
- Escape sequences when using single quotes for strings:
  - to include an apostrophe, use \'
  - to include a backslash, just use \, but also \\
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.";
```

Long or fiddly

This is painfully long:

echo '<p>';
echo 'Hello ';
echo $firstname; 
echo ' ';
echo $surname;
echo '</p>';

This is painfully fiddly:

echo '<p>Hello ' . $firstname . ' ' . $surname . '</p>';

Interpolation

With double quotes, you can use interpolation:
```
echo "<p>Hello {$firstname} {$surname}</p>";
```
Interpolation: in strings written using double quotes, variables in braces are replaced by their values
Notes:
- This only works with double quotes
- In fact the braces are optional most of the time but we will always use them

Class exercise: What is the output from each of the following?

$answer = 'forty-two';

echo 'The answer is ' . $answer;
echo 'The answer is {$answer}';
echo 'The answer is $answer';
echo "The answer is " . $answer;
echo "The answer is {$answer}";
echo "The answer is $answer";

Which should I use?

Single quotes
- A teeny bit faster than using double quotes
- Use single quotes if you don't need interpolation
- Use single quotes if your string contains double quotes
Double quotes
- The opposite to the above

PHP: Strings

Derek Bridge

Department of Computer Science, University College Cork

PHP: Strings

Initialisation

Class exercise I: what happens?

Class exercise II: what happens?

Class exercise III: what happens?

Class exercise IV: what happens?

Expressions, operators and operands

String expressions

Class exercise: What is the output from each of the following?

Using string concatenation in programs

Single quotes and double quotes

Long or fiddly

Interpolation

Class exercise: What is the output from each of the following?

Which should I use?

Department of Computer Science,
University College Cork