PHP syntax, statements and comments

Statements and semicolons

A PHP program is a list of statements: instructions that PHP carries out one after the other, from the top of the file to the bottom. Each statement ends with a semicolon.

<?php
echo "First\n";
echo "Second\n";
echo "Third\n";

First
Second
Third

The semicolon is not optional. It is how PHP knows where one instruction stops and the next begins. Line breaks do not matter to PHP, which is why two statements can share a line, although that makes code harder to read:

<?php
echo "One "; echo "line\n";

One line

Blocks and braces

Statements that belong together are grouped in a block with curly braces { and }. You will see blocks after if, loops, functions and classes:

<?php
$stock = 0;
if ($stock === 0) {
    echo "Out of stock\n";
    echo "Check back next week\n";
}

Out of stock
Check back next week

The indentation (four spaces is the common standard, called PSR-12) is for people, not for PHP. PHP only looks at the braces. Consistent indentation makes it obvious which statements are inside which block, so keep it tidy from the start.

Case sensitivity

PHP is partly case-sensitive, which surprises people:

• Variable names are case-sensitive. $price and $Price are two different variables.
• Keywords and function names are not. echo, ECHO and Echo all work, and so do strlen() and STRLEN().

<?php
$price = 10;
$Price = 99;
ECHO "price is $price, Price is $Price\n";
echo strtoupper("works") . "\n";

price is 10, Price is 99
WORKS

Even though uppercase keywords work, everyone writes them in lowercase. Use lowercase keywords and pick one style for variable names (this course uses $camelCase, such as $orderTotal) so you never have two names that differ only by case.

Comments

Comments are notes for people reading the code. PHP ignores them completely. There are three forms:

<?php
// A single-line comment: everything after // on this line is ignored.
$note = "x"; # The hash sign also starts a single-line comment.

/*
   A block comment can span
   several lines.
*/

echo "Comments do not print anything\n"; // a comment can also follow code

Comments do not print anything

A fourth form, the doc comment, starts with /**. It is still a comment to PHP, but editors and documentation tools read it to show what a function expects and returns:

<?php
/**
 * Adds 18% tax to a price.
 *
 * @param float $price The price before tax.
 * @return float The price including tax.
 */
function withTax(float $price): float
{
    return $price * 1.18;
}

echo withTax(100) . "\n";

118

Good comments explain why the code does something, not what it obviously does. $total = $total * 1.18; // add 18% GST is helpful. $i = $i + 1; // add one to i is noise.

Comments are also handy for switching a line off while you experiment: put // in front of it instead of deleting it.

Syntax errors

If you break a rule, for example by forgetting a semicolon, PHP cannot understand the file and refuses to run any of it. This file is missing the semicolon after the first echo:

<?php
echo "Before the mistake\n"
echo "After the mistake\n";

Parse error: syntax error, unexpected token "echo", expecting "," or ";" in /path/to/script.php on line 3

Two things to learn from this message. First, the line number points to where PHP noticed the problem, which is often the line after the real mistake. Second, not even the first echo ran: PHP reads (parses) the whole file before running it. When you see "unexpected", look at the end of the previous line first.

On a development machine the message appears in the output, as above. A production server should hide it from visitors and write it to a log file instead, a setting covered in Errors and exceptions.

Whitespace

Spaces, tabs and blank lines between tokens are ignored, so $total=$a+$b; and $total = $a + $b; are identical to PHP. The second is far easier to read. Put spaces around operators, one statement per line, and a blank line between groups of related statements.

Common mistakes

• Missing semicolons, especially at the end of the line before an error message.
• Unbalanced braces: every { needs a matching }. An editor that highlights matching braces helps.
• Smart quotes pasted from a word processor or chat app (“ instead of "). PHP does not treat them as quotes.
• Using HTML comments inside PHP code. <!-- --> is only a comment in the HTML part of a file.

Exercise

The script below has three problems. Find and fix them, then run it. It should print Total: 30.

<?php
$a = 10
$b = 20;
// add the numbers
$total = $a + $B;
echo "Total: $total\n";

<?php
$a = 10;
$b = 20;
// add the numbers
$total = $a + $b;
echo "Total: $total\n";

Total: 30