2.12.2.1. Variable naming

Making a mistake with a variable name sometimes inadvertently creates never-ending loops. The result of a never-ending loop is that the web browser eventually times out and alerts the user that the web server isn't responding to an HTTP request.

The following loop never ends, and no output is produced:

for($counter=0; $counter<10; $Counter++)
  myFunction( );

The variable $counter is never incremented. Instead, another variable, $Counter, is, so $counter is always less than 10. Common bugs result from subtle changes in variable names through changing case, omitting or including underscores, or simple typographic errors.

Never-ending loops can also produce unexpected output. The following loop can render thousands of greetings in a web browser in a very short time:

for($counter=0; $Counter<10; $counter++)
  echo "<br>hello";

These errors can sometimes be detected by setting the PHP error-reporting level to a higher sensitivity. Adding the following code fragment to the top of each PHP script or to a file included with the require directive reports undefined variable errors:

error_reporting(E_ALL);

This forces variables to be declared by assigning a value before they can be used. Consider the following example:

error_reporting(E_ALL);
for($counter=0; $Counter<10; $counter++)
  echo "<br>hello";

This produces an unending number of warning messages stating:

Warning: Undefined variable: Counter in /var/lib/apache/htdocs/winestore/a.php on line 2

2.12.2.2. Missing output

$testvariable = "hello";
echo "The value of test is $testVariable";

This produces the string:

The value of test is 

The problem can be much harder to identify by visual inspection if the variable is part of a complex operation, such as being used as an array element index, part of <table> output, or as a parameter to a database query.

If output appears but isn't as expected, an uninitialized variable is a possibility. The simplest approach to detecting the error is then to check for a bug by setting error_reporting(E_ALL) at the top of the script as discussed in the last section.

Another related problem involves variable names appearing where values should. This is usually the simple problem of an omitted dollar sign and is easy to fix. For example:

echo "the value of test is test";

This should have been:

echo "the value of test is $test";

If a dollar sign is omitted in a statement such as an assignment or conditional, the PHP interpreter reports a specific parse error with its default error-reporting level.

A similar problem can also occur when single quotes are used instead of double quotes, because single-quoted strings are always output directly, and the string isn't interpreted like a double-quoted string is. For example:

echo 'the value of test is $test';

This produces:

the value of test is $test

It doesn't output the value of the variable $test.