合并变量PHP

im confuse that...

like example:

$Q1 = "hello";
$Q2 = "world";
$Q3 = "StackOverflow";

$i = 1;

while($i < 3) {
  $a = "$Q".$i; //I think this is wrong.    
  echo $a; // i tried ${$a} doesn't work =/

  $i++;
}

then output format:

$Q1
$Q2
$Q3

but there is not output like this: hello world StackOverflow

I want like $Q + $i become $Q1 to answer is: "hello"...

$varName = 'Q'.$i;
$a .= $$varName; 

Or just

echo $$varName . "<br>
";

echo $Q1 . $Q2 . $Q3; will output what you're looking for.

Alternatively, you could do this:

$a = '';

for($i = 1; $i <= 3; $i++)
  $a .= ${'Q' . $i};

echo $a;

To create the variable variable, use:

$a = ${'Q'.$i};

Yeah. When you have double quoted strings, and you put a dollar sign and something else in it, it interprets it as a variable. (it also escape things like )

Example

$test = "hi";
echo "$test world"; //This outputs hi world

In your case, $Q doesn't exist. The default PHP behaviour is to ignore that error and just puts out nothing. This is why it's recommended to report all problems with your code. Check out http://php.net/manual/en/function.error-reporting.php for details.

Solution to your problem would be using single quoted strings. do $a = '$Q'.$i;

What you are doing there is simply printing the string '$Q1', '$Q2' and '$Q3'. In PHP you use dynamic variable names this way:

<?php
$Q1 = 'hello';
$Q2 = 'world';
$Q3 = 'StackOverflow';
for ($i = 1; $i <= 3; $i++) {
    echo ${'Q' . $i};
}
?>

$Q = array("hello", "world", "StackOverflow");
foreach($Q as $w) {
    echo $w;
}

If you can't do something like this then you will need to use dynamic variables:

$var = 'Q' . $i;
echo $var; 

PHP does support variable variable names, denoted using $$. This will do what you want.

$qvar = 'Q'.$i;
$a = $$qvar;

However, this is considered very poor practice -- almost as bad as using eval() (and for similar reasons).

The correct answer would be to create an array of $Q, and referencing array elements;

$Q = array(
        "hello",
        "world",
        "StackOverflow")

$a = $Q[0] . $Q[1] . $Q[2];