PHP - 打印布尔表达式的结果

I expect the following code to print "false"

<?php
    $a = 4;
    $b = 45;
    echo $a==$b;
?>

but prints nothing.

It doesn't print anything because the result of $a==$b is a boolean, and false is converted to an empty string. Use var_dump instead (if you are debugging your code):

var_dump($a==$b);

Or alternatively you can use echo this way:

echo ($a==$b) ? 'true' : 'false';