Ask a function in the IF statement about the problem, I am a beginner

Ask a function in the IF statement problem, I rookie a
If there is a similar function, incomplete, just an example.

function p_abc ($a = ', $b = ', $c = ') {
$a = xxx
if ($b = ' 255 ') {
$a =xxx+1
}
$c =yyy

}


When $b = ' 255 ', the function works properly, and when $b is not equal to 255, it seems $c =yyy cannot execute (I'm not sure). Causes an error in the program.

Why is it? I rookie one, good-hearted people tell.

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------Solution--------------------
A = is an assignment
Two = is a comparison

you if ($b = ' 255 ') {is to make $b 255, the expression is always set
should be written if ($b = =' 255 ') {

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