PHP tags
I recommend always using the full PHP open tag, <?php, rather than the short one, <?. This means also that instead of <?=$var?>, it's better to use the full <?php echo $var?>. Using the full open tag ensures that your code will work on all PHP installations, regardless of the short_open_tag setting.


Alternative control structure syntax
I highly recommend staying away from the if (): ... endif; style syntax and sticking with curly braces, if () { ... }. This is the preferred syntax and it makes your code easier to read.


error_reporting
You should probably test your code with error_reporting set to at least E_ALL & ~E_NOTICE (all errors except notices) so that you aren't suppressing errors that should be fixed. error_reporting can be set in php.ini or with the error_reporting() function.


Line breaks
People want to know how they can retain textarea line breaks in HTML. You should store text in the database in its original format (e.g. with just newlines) and then use nl2br() to convert newlines to HTML <br /> tags on display. That's all good, except for one problem with nl2br(): it doesn't seem to convert \r newlines (edit: this has now been fixed in PHP 4.2.0).

Windows uses \r\n newlines; *nix uses \n; Mac uses \r.

nl2br() works correctly on text from Windows/*nix because they contain \n. However, if you get text from a Mac, nl2br() will not convert its newlines (again, fixed in PHP 4.2.0). To remedy this, I use the following bit of code to convert \r\n or \r to \n before inserting it into the database. It won't hurt anything and ensures that nl2br() will work on the \n only newlines on display. Also, it has the side effect of saving 1 byte in the database per newline from Windows (by storing only \n instead of \r\n).


Strings
1) As a finesse thing, I use single quotes around strings whenever possible (e.g. strings that don't contain variables, single quotes, \n, etc.). This is supposed to make less work for the PHP parser.

2) When an array variable isn't in a string, put quotes around string-literal keys so they are not regarded as constants:

3) Remember, you can break out of PHP mode for large sections of HTML. This is faster than echo'ing and you don't need to escape quotes.


Quotes around numeric data in queries
For numeric columns in MySQL, you shouldn't put quotes around any of their values in queries. As our resident database guru, MattR, says, "that is very non-standard and will only work on MySQL." But if it's unknown data, how do you know that it's numeric and not letters that will cause an error? You can make sure that only a number is used in the query by first type-casting the data as int (or float for decimal numbers):

Then even if id is set to "abc," the worst that can happen is a 0 will be used in the query. No quotes; no error.