Why does (0 == 'Hello') return true in PHP?

Question

Hey, if you have got the following code and want to check if $key matches Hello I've found out, that the comparison always returns true if the variable is 0. I've came across this when an array for a special key and wondered why it's wasn't working as expected. See this code for an example.

$key = 1; if ($key != 'Hello') echo 'Hello'; //echoes hello  $key = 2; if ($key != 'Hello') echo 'Hello'; //echoes hello  $key = 0; if ($key != 'Hello') echo '0Hello'; //doesnt echo hello. why? if ($key !== 'Hello') echo 'Hello'; //echoes hello 

Can anyone explain this?

Answer 1

The operators == and != do not compare the type. Therefore PHP automatically converts 'Hello' to an integer which is 0 (intval('Hello')). When not sure about the type, use the type-comparing operators === and !==. Or better be sure which type you handle at any point in your program.

Answer 2

Others have already answered the question well. I only want to give some other examples, you should be aware of, all are caused by PHP's type juggling. All the following comparisons will return true:

• 'abc' == 0
• 0 == null
• '' == null
• 1 == '1y?z'

Because i found this behaviour dangerous, i wrote my own equal method and use it in my projects:

/**  * Checks if two values are equal. In contrast to the == operator,  * the values are considered different, if:  * - one value is null and the other not, or  * - one value is an empty string and the other not  * This helps avoid strange behavier with PHP's type juggling,  * all these expressions would return true:  * 'abc' == 0; 0 == null; '' == null; 1 == '1y?z';  * @param mixed $value1  * @param mixed $value2  * @return boolean True if values are equal, otherwise false.  */ function sto_equals($value1, $value2) {   // identical in value and type   if ($value1 === $value2)     $result = true;   // one is null, the other not   else if (is_null($value1) || is_null($value2))     $result = false;   // one is an empty string, the other not   else if (($value1 === '') || ($value2 === ''))     $result = false;   // identical in value and different in type   else   {     $result = ($value1 == $value2);     // test for wrong implicit string conversion, when comparing a     // string with a numeric type. only accept valid numeric strings.     if ($result)     {       $isNumericType1 = is_int($value1) || is_float($value1);       $isNumericType2 = is_int($value2) || is_float($value2);       $isStringType1 = is_string($value1);       $isStringType2 = is_string($value2);       if ($isNumericType1 && $isStringType2)         $result = is_numeric($value2);       else if ($isNumericType2 && $isStringType1)         $result = is_numeric($value1);     }   }   return $result; } 

Hope this helps somebody making his application more solid, the original article can be found here: Equal or not equal