Question

I came across this code in a php database class:

if( !$this->_Link_ID )

Link_ID is an integer.

So does this code just check if Link_ID is not 0?

I know from experience that if a variable is type Boolean, you can just test the var like

$myBoolean = true;
if ($myBoolean){
    // code
}

I didn’t realise this can be done for integers.

So how is if( !$this->_Link_ID ) evaluated?

Answer 1

It checks if the integer is zero if it’s integer. It also evaluates to truth if it’s set to null and if it’s unset, but in the latter case it also spits out a warning. if there was no negation, that would be a test for non-zero.

For more details see: converting to boolean:

When converting to boolean, the following values are considered FALSE:

• the boolean FALSE itself
• the integer 0 (zero)
• the float 0.0 (zero)
• the empty string, and the string “0”
• an array with zero elements
• an object with zero member variables (PHP 4 only)
• the special type NULL (including unset variables)
• SimpleXML objects created from empty tags

Every other value is considered TRUE (including any resource).