Continued from Understanding Variables:
PHP variables must also conform to a set of rules. Variable names must start with a dollar sign. They can contain any combination of strings, numbers, and underscores, but the first character after the dollar sign cannot be a number. PHP variables are also case-sensitive.
PHP variables don’t need an initial value when they are created, and they don’t need to be declared a specific data type either.
PHP has several built-in data types:
- boolean – a data type with only two possible values: true or false
- integer – any positive or negative whole number, or a zero
- float – numbers that are too large or too small to be represented as integers
- string – a series of alphanumeric characters, numbers, and punctuation marks
- array – a data type which can hold several different values
- object – built-in or user-defined classes of data; a collection of properties or attributes
- resource – a variable holding a reference to an external resource, like a MySQL database
- NULL – a special data value that has no value and means nothing; not the same as zero
When using integers, a leading 0 ( zero ) is used to specify that the integer is octal, and a leading 0x or 0X is used to specify hexadecimal. A floating point number can contain either a decimal or an “e” to represent “ten to the power of” in scientific notation, or both.