PHP变量没有显示变化？

Question

When I change individual characters in PHP, and then echo those changes out, the changes are not displayed, and the original value of the variable shows up. Here is my code:

<?php
function encrypt($string) {
$string_change = $string;
for ($i = 0; $i < count($string_change); $i++) {
    if ($string_change[$i] == 'a') $string_change[$i] == '6';
    else if ($string_change[$i] == 'b') $string_change[$i] == '3';
    else if ($string_change[$i] == 'c') $string_change[$i] == '7';
    else if ($string_change[$i] == 'd') $string_change[$i] == '6';
    else if ($string_change[$i] == 'e') $string_change[$i] == '2';
    else if ($string_change[$i] == 'f') $string_change[$i] == '5';
    else if ($string_change[$i] == 'g') $string_change[$i] == '8';
    else if ($string_change[$i] == 'h') $string_change[$i] == '9';
    else if ($string_change[$i] == 'i') $string_change[$i] == '1';
    else if ($string_change[$i] == 'j') $string_change[$i] == '0';
    else if ($string_change[$i] == 'k') $string_change[$i] == '!';
    else if ($string_change[$i] == 'l') $string_change[$i] == '%';
    else if ($string_change[$i] == 'm') $string_change[$i] == '*';
    else if ($string_change[$i] == 'n') $string_change[$i] == '$';
    else if ($string_change[$i] == 'o') $string_change[$i] == '(';
    else if ($string_change[$i] == 'p') $string_change[$i] == '&';
    else if ($string_change[$i] == 'q') $string_change[$i] == '^';
    else if ($string_change[$i] == 'r') $string_change[$i] == ')';
    else if ($string_change[$i] == 's') $string_change[$i] == '-';
    else if ($string_change[$i] == 't') $string_change[$i] == '+';
    else if ($string_change[$i] == 'u') $string_change[$i] == '_';
    else if ($string_change[$i] == 'v') $string_change[$i] == '=';
    else if ($string_change[$i] == 'w') $string_change[$i] == '[';
    else if ($string_change[$i] == 'x') $string_change[$i] == '{';
    else if ($string_change[$i] == 'y') $string_change[$i] == '}';
    else if ($string_change[$i] == 'z') $string_change[$i] == ']';
    else if ($string_change[$i] == ' ') $string_change[$i] == '/';
}
return $string_change;
}
function decrypt($string) {
$string_change = $string;
for ($i = 0; $i < count($string_change); $i++) {
    if ($string_change[$i] == '6') $string_change[$i] == 'a';
    else if ($string_change[$i] == '3') $string_change[$i] == 'b';
    else if ($string_change[$i] == '7') $string_change[$i] == 'c';
    else if ($string_change[$i] == '6') $string_change[$i] == 'd';
    else if ($string_change[$i] == '2') $string_change[$i] == 'e';
    else if ($string_change[$i] == '5') $string_change[$i] == 'f';
    else if ($string_change[$i] == '8') $string_change[$i] == 'g';
    else if ($string_change[$i] == '9') $string_change[$i] == 'h';
    else if ($string_change[$i] == '1') $string_change[$i] == 'i';
    else if ($string_change[$i] == '0') $string_change[$i] == 'j';
    else if ($string_change[$i] == '!') $string_change[$i] == 'k';
    else if ($string_change[$i] == '%') $string_change[$i] == 'l';
    else if ($string_change[$i] == '*') $string_change[$i] == 'm';
    else if ($string_change[$i] == '$') $string_change[$i] == 'n';
    else if ($string_change[$i] == '(') $string_change[$i] == 'o';
    else if ($string_change[$i] == '&') $string_change[$i] == 'p';
    else if ($string_change[$i] == '^') $string_change[$i] == 'q';
    else if ($string_change[$i] == ')') $string_change[$i] == 'r';
    else if ($string_change[$i] == '-') $string_change[$i] == 's';
    else if ($string_change[$i] == '+') $string_change[$i] == 't';
    else if ($string_change[$i] == '_') $string_change[$i] == 'u';
    else if ($string_change[$i] == '=') $string_change[$i] == 'v';
    else if ($string_change[$i] == '[') $string_change[$i] == 'w';
    else if ($string_change[$i] == '{') $string_change[$i] == 'x';
    else if ($string_change[$i] == '}') $string_change[$i] == 'y';
    else if ($string_change[$i] == ']') $string_change[$i] == 'z';
    else if ($string_change[$i] == '/') $string_change[$i] == ' ';
}
return $string_change;
}
?>

Can anyone offer a suggestion as to how to change this so that the changes show up?