php why does TRUE == "expired"?

March 26, 2022

This drove me nuts for a few hours.

I have a function returning one of the three following values:

function checkValid() {
   ...
   return array("expired",$oldDate,$newDate) ;
   return array(true,$oldDate,$newDate) ;
   return array(false,$oldDate,$newDate) ;
}

list($isValid,$oDate,$nDate) = checkValid() ;

if ($isValid == "expired") {
   ... 
   ...do blah
}

…and everytime the condition returned true, the if ($isValid == "expired") { ... } would trigger. So I ran some tests and sure enough:

$isValid = true ;
if ($isValid == "expired") {
   echo "Yup...some how 'expired' equals TRUE" ;
} else {
   echo "Nope....it doesn't equal true" ;
}

output:  Yup...some how 'expired' equals TRUE

When I changed the if/condition to:

$isValid = true ;
if ($isValid === "expired") {
   echo "Yup...some how 'expired' equals TRUE" ;
} else {
   echo "Nope....it doesn't equal true" ;
}

output: Nope....it doesn't equal true

I am baffled by this. Why would true == 'expired' or 1 == 'expired' ???

>Solution :

When using two equal signs == php does type coersion under the hood and checks for truthy cases which includes all numbers other than 0, boolean true, all string other than empty strings and some other cases.

If you want to check for an exact match, you should use three equal signs ===