WMPRO, WMMINI FW >= 1.0 WMMEGA FW >= 2.0

Check if a variable exists

Determine if a variable is set

$variable: Variable to check for existence¹⁾

Integer: 1 (true) if variable exists, 0 (false) if variable does not exist

<?
  print(isset($x)); // outputs 0, the variable doesn't exist yet
  $x=0;
  print(isset($x)); // outputs 1
 
  $y='';
  print(isset($y)); // outputs 1, $y exists (although it is an empty string)
 
  $z=chr(0);
  print(isset($z)); // outputs 1, $z exists with a single character (ASCII code 0 or NUL)
 
  $array=array();
  print(isset($array)); // outputs 1, $array exists (an empty array)
 
  $array[1]="test";
  print(isset($array[0])); // outputs 0, the first element in the array does not exist
  print(isset($array[1])); // outputs 1, the second element in the array exists
?>

There is no way to unset a variable once it has been set (except to exit the local scope or script where it was defined). Unlike mainline PHP, uPHP does not have a construct unset() or the constant null. Unlike many other programming languages, you cannot simply remove a variable by setting it equal to nothing, such as by trying the statement $x=;²⁾.

Also note that the uPHP isset() does not always generate a parsing error if the parameter is not a variable³⁾. All of these examples will return 0:

<?
  print(isset(1));
  print(isset("123"));
  print(isset(array(1,2,3)));
  print(isset(2+3));
?>

Although maybe useful in some situations, it is better not to use isset() for expressions or values instead of variables, because in some cases it will generate a parsing error. Example:

<?
  print(isset(z));
?>

For the above example the parser answers:

Error on line 1: Undefined constant z
Error on line 1: Invalid function

Also note that in many situations where using if to test for the existence of a variable it is possible to simplify the conditional test by using if($x) instead of if(isset($x)) but only if you are certain that $x does not contain the value 0 (false). Unlike mainline PHP, uPHP does not generate an error for using if($x) on an undefined variable. Nevertheless, this is not a good practice even though this hack is commonly used somewhat successfully by Wattmon uPHP programmers. It is much better to use isset() if you are not certain that the variable has been initialized, and the code will be easier to understand later on.

is_int() - Check if a variable is an integer

is_float() - Check if a variable is a float

is_numeric() - Check if a value is numeric (int, float or numeric string)

is_array() - Check if a variable is an array

is_string() - Check if a variable is a string

uPHP Variable Types and Limits