PHP basics

Sun Feb 18 17:11:16 EST 2007

This is a small summary of a small part of PHP; it shows mostly the
common things and a few that I have trouble remembering.  I am not a PHP
user; caveat lector.

The CS machines are running PHP 4; PHP 5 is a better choice because
it supports object oriented programmer better.

Program structure: ============================================

PHP was derived from Perl, at least in spirit, so it shares some of the
syntactic rules and oddities, though it's a lot simpler.  One major
difference: PHP code has to be enclosed within <?php ... ?>; when
processed by a web server, anything outside those delimiters is just
sent back to the client, while anything within the delimiters is
processed by PHP and the result is included in what goes to the client.
When used standalone, it looks like anything outside the delimiters is
just ignored, though there is probably some option to make it just read
code and run it.

Scalar variable names begin with $, as in Perl.  Variables are typeless.
You can add a number and a string, and the result is to take the prefix
of the string that is numeric and use that.

Variables are not declared except by initializing them, or by a global
declaration within a function to state that a variable is external;
otherwise variables are local to their functions.

Variables need not be initialized; they have a null (empty string)
value.

There are some built-in variables, like those that come back from a web
form, that are global or even super-global.  The most useful are
$_SERVER, $_GET and $_POST, which are arrays of the values returned from
the web page; each of these is an array so the subscripts are names like
HTTP_REFERRER.

String constants are quoted with '...' or "..."; backslashes are
interpreted within strings, as are $references to variables.

Arrays can have numeric and/or string subscripts; there does not seem to
be a distinction between indexed and associative arrays.  Set array
values with
	$a[index] = value
and elements are accessed with $a[index] where indices run from 0 to
count($a).  Add new elements at the end with $a[] = whatever.

Relational operators are more or less as in C, but there are both && ||
and "and" "or"; the latter have lower priority, for things like
	do something or die()

It looks like the same operator for string and numeric comparisions,
but there are === and !== operators that test for equality of type and
content.

Control flow is like Perl in most respects, except that braces are not
needed around a body consisting of a single statement.  Semicolons are
needed at the end of each statement, but you don't need braces {} for
a single statement as in Perl.

	if (whatever)
		...
	elseif (...)
		...
	else
		...

	while (expression)
		...

	do ... while (expression)

	for (init; expression; re-init)
		...

	foreach ($array as $value) ...
		loops over the values in the array
	foreach ($array as $key => $value) ...
		loops over the values, also providing the keys

	switch (expression) {
		case ...:
		default:
	}

Within a loop,
	break, as in C
	continue, as in C

Functions are declared without arguments and can appear anywhere:

	function foo(args) {
		variables are local unless
		global v1, v2
		return whatever
	}

Arguments are passed by value (including arrays?   I can't tell)
though there is a & operator that means call by reference.


=============================================================

You should be able to run
	php -r '...'
in standalone mode; you don't need <? ?> and there's no spurious output
at the end.

The little text formatter:

$line = '';
$space = '';

#$rh = fopen($argv[1], "r");
#if ($rh === false) {
#	echo "can't open file " . $argv[1] . "\n";  # ' vs " matters
#	return;
#}

$rh = STDIN;
while (!feof($rh)) {
	$d = rtrim(fgets($rh));
	if (strlen($d) == 0) {
		printline();
		print "\n";
	} else {
		#$words = split("/[\s]+/", $d); 
			# slashes needed!  doesn't work in 4.3.11
		$words = explode(" ", $d); 
			# regular string split doesn't handle mult blanks
		$c = count($words);
		for ($i = 0; $i < $c; $i++)
			if (strlen($words[$i]) > 0)
				addword($words[$i]);
	}
}
fclose($rh);
printline();

function addword($w) {
	global $line, $space;
	if (strlen($line) + strlen($w) > 60)
		printline();
	$line .= $space . $w;
	$space = ' ';
}

function printline() {
	global $line, $space;
	if (strlen($line) > 0)
		print "$line\n";
	$line = '';
	$space = '';
}
# the \n after the next line shows up in the output!!
?>

The stdin etc file handles are there as STDIN, STDOUT, STDERR.

Regular expressions have to be surrounded by slashes even if they are
provided by strings for functions like preg_match.

I had a lot of trouble with this, and some things still don't work,
which suggests that my understanding is imperfect.