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A typical problem is the following: when a user has the viewport at a size that makes the outer container 999 pixels wide, if the first column is 60% and the second is 40%, IE 6 will always calculate the two columns as 600 and 400 pixels and as a result, the two will not fit (600+400 = 1 more than 999) and it will drop the second column. This is obviously not intended behavior, and in a world where we still have to use floats for columns (I can't wait for display:table support across all browsers), it's important to work around this problem. I used to give my last column 1 less percent (in this example, it would have 39% instead of 40%, but this would usually result in columns that don't quite fill up the container. Of late I have been giving the last column .4 less percent (in this example, 39.6%), which seems to work perfectly. Browsers will calculate this width and round up, but it will still fit even with an odd container width like 999px and I won't have to worry about dropped columns.

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.