What is the question mark in the middle of a php expression?

The question mark in the middle of a php expression is a shorthand for a conditional statement and comes together with a column. Together they form a ternary operator.

isset($x)? $y:$z

The expression above evaluates to $y if isset($x) is true and to $z if it is false. Note that isset($x) can be any function method or variable returning a boolean.