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PHP also supports two composite (non-scalar) types: arrays and objects. Each of these value types can be assigned into variables or returned from functions.

Since assignments are parsed in a right to left order, you can also write '$b = $a = 5'

A very common type of expressions are comparison expressions. These expressions evaluate to either FALSE or TRUE. PHP supports > (bigger than), >= (bigger than or equal to), == (equal), != (not equal), < (smaller than) and <= (smaller than or equal to). The language also supports a set of strict equivalence operators: === (equal to and same type) and !== (not equal to or not same type). These expressions are most commonly used inside conditional execution, such as if statements.

The last example of expressions we'll deal with here is combined operator-assignment expressions

Adding 3 to the current value of $a can be written '$a += 3'

There is one more expression that may seem odd if you haven't seen it in other languages, the ternary conditional operator:

Some expressions can be considered as statements. In this case, a statement has the form of 'expr ;' that is, an expression followed by a semicolon. In '$b = $a = 5;', '$a = 5' is a valid expression, but it's not a statement by itself. '$b = $a = 5;' however is a valid statement.

One last thing worth mentioning is the truth value of expressions. In many events, mainly in conditional execution and loops, you're not interested in the specific value of the expression, but only care about whether it means TRUE or FALSE. The constants TRUE and FALSE (case-insensitive) are the two possible boolean values.

Throughout the rest of this manual we'll write expr to indicate any valid PHP expression.