Weird PHP String Integer Comparison and Conversion - php

Question

I was working on some data parsing code while I came across the following.
$line = "100 something is amazingly cool";
$key = 100;
var_dump($line == $key);
Well most of us would expect the dump to produce a false, but to my surprise the dump was a true!
I do understand that in PHP there is type conversion like that:
$x = 5 + "10 is a cool number"; // as documented on PHP manual
var_dump($x); // int(15) as documented.
But why does a comparison like how I mentioned in the first example converts my string to integer instead of converting the integer to string.
I do understand that you can do a === strict-comparison to my example, but I just want to know:
Is there any part of the PHP documentation mentioning on this behaviour?
Can anyone give an explanation why is happening in PHP?
How can programmers prevent such problem?

Answer 1

If I recal correcly PHP 'casts' the two variables to lowest possible type.
They call it type juggling.
try: var_dump("something" == 0);
for example, that'll give you true . . had that bite me once before.
More info: http://php.net/manual/en/language.operators.comparison.php

Answer 2

I know this is already answered and accepted, but I wanted to add something that may help others who find this via search.
I had this same problem when I was comparing a post array vs. keys in a PHP array where in my post array, I had an extra string value.
$_POST["bar"] = array("other");
$foo = array(array("name"=>"foobar"));
foreach($foo as $key=>$data){
$foo[$key]["bar"]="0";
foreach($_POST["bar"] as $bar){
if($bar==$key){
$foo[$key]["bar"]="1";
}
}
}
From this you would think that at the end $foo[0]["bar"] would be equal to "0" but what was happening is that when $key = int 0 was loosely compared against $bar = string "other" the result was true to fix this, I strictly compared, but then needed to convert the $key = int 0 into a $key = string "0" for when the POST array was defined as array("other","0"); The following worked:
$_POST["bar"] = array("other");
$foo = array(array("name"=>"foobar"));
foreach($foo as $key=>$data){
$foo[$key]["bar"]="0";
foreach($_POST["bar"] as $bar){
if($bar==="$key"){
$foo[$key]["bar"]="1";
}
}
}
The result was $foo[0]["bar"]="1" if "0" was in the POST bar array and $foo[0]["bar"]="0" if "0" was not in the POST bar array.
Remember that when comparing variables that your variables may not being compared as you think due to PHP's loose variable typing.