What happens when you adda string to an integer in PHP?

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PHP

i got this line of code

$a=5;
$b="77ff";
$c=$a+$b;
echo $c;

it prints an error message once i run the code but still it adds the value of the 'a' to the number appears at the beginning of variable 'b' and then it prints 'c'

What I have tried:

ran the code with different string values checked that it only adds string to a number only if the string started with a numeric value is there a specific reason for that to happen or is it just how the php works?