PHP does not require (or support) explicit type definition in variable declaration; a variable's type is determined by the context in which that variable is used. That is to say, if you assign a string value to variable var, var becomes a string. If you then assign an integer value to var, it becomes an integer.
An example of PHP's automatic type conversion is the addition operator '+'. If any of the operands is a double, then all operands are evaluated as doubles, and the result will be a double. Otherwise, the operands will be interpreted as integers, and the result will also be an integer. Note that this does NOT change the types of the operands themselves; the only change is in how the operands are evaluated.
$foo = "0"; // $foo is string (ASCII 48)
$foo++; // $foo is the string "1" (ASCII 49)
$foo += 1; // $foo is now an integer (2)
$foo = $foo + 1.3; // $foo is now a double (3.3)
$foo = 5 + "10 Little Piggies"; // $foo is integer (15)
$foo = 5 + "10 Small Pigs"; // $foo is integer (15)
If the last two examples above seem odd, see String conversion.
If you wish to force a variable to be evaluated as a certain type, see the section on Type casting. If you wish to change the type of a variable, see settype().
Type casting in PHP works much as it does in C: the name of the desired type is written in parentheses before the variable which is to be cast.
$foo = 10; // $foo is an integer
$bar = (double) $foo; // $bar is a double
The casts allowed are:
(int), (integer) - cast to integer
(real), (double), (float) - cast to double
(string) - cast to string
(array) - cast to array
(object) - cast to object
Note that tabs and spaces are allowed inside the parentheses, so the following are functionally equivalent:
$foo = (int) $bar;
$foo = ( int ) $bar;