String Plus number Why the result is a number (interview question)

A few days ago to participate in an interview, out of a problem, write the results of the operation, the topic is probably a string plus a number is assigned to a variable, output variable:
$test = ' Linux ' + 6;
Echo $test;
Later run to learn that the results are actually behind the numbers, but still do not understand why this is, for guidance, thank you.

Reply to discussion (solution)

$test = ' Linux ' + 6;
You're doing an addition!
Since it's an addition, the string will be converted to a number to participate in the operation.
Strings that start with a number other than numbers can only be converted to 0
That
$test = ' Linux ' + 6;
Equivalent to
$test = 0 + 6;
So Echo $test; The output is 6.

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