我怎样才能在php中实现FibonacciSecret

Your Fibonacci series generation code was not correct, Try this:

Update:

This code will stop once it sum goes out of the range of message length.

$message = 'The Da Vinci Code is a 2003 mystery-detective novel by Dan Brown';

// remove all the spaces from message
$message = str_replace(' ', '', $message);
$str_split = str_split($message);

$x = 0;    
$y = 1;
$next = 0;

//stopping the loop if the character index goes out of range
for($i=1;$next<=count($str_split);$i++)  
{      
    if($i == 1) //for first element use 0 as the sum
    {
        $farray[] = $x; 
        continue;
    }
if($i == 2) //for second element use 1 as the sum
{
     $farray[] = $y; 
    continue;
}

$next = $x + $y;
$x=$y;
$y=$next;

$farray[] = $next; 
}  

foreach($farray as $key=>$fvalue){
   echo $str_split[$fvalue]."--";
}

Original Answer:-

It Works:-

$message = 'The Da Vinci Code is a 2003 mystery-detective novel by Dan Brown';

// remove all the spaces from message
$message = str_replace(' ', '', $message);
$str_split = str_split($message);

$x = 0;    
$y = 1;
$next = 0;

for($i=1;$i<=10;$i++)    
{      
    if($i == 1) //for first element use 0 as the sum
    {
        $farray[] = $x; 
        continue;
    }

    if($i == 2) //for second element use 1 as the sum
    {
         $farray[] = $y; 
        continue;
    }

    $next = $x + $y;
    $x=$y;
    $y=$next;

    $farray[] = $next; 
}  

foreach($farray as $key=>$fvalue){
   echo $str_split[$fvalue]."--";
}

I think your Fibonacci logic is off. I munged up an example I found here: https://www.easycalculation.com/code-php-program-fibonacci-series.html

Sequence: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34;

<?php
$message = 'The Da Vinci Code is a 2003 mystery-detective novel by Dan Brown';
$str_split = str_split(str_replace(' ', '', $message));

$n_value = 10;
$first_value = 0;
$second_value = 1;
$next_value = 0;
$c_value = 0;
$letters = [];

for ( $c_value = 0 ; $c_value < $n_value ; $c_value++ )
{
    if ( $c_value <= 1 ) {
         $next_value = $c_value;
    } else {
         $next_value = $first_value + $second_value;
         $first_value = $second_value;
         $second_value = $next_value;
    }
    $letters[] = $str_split[$next_value];
}

echo implode($letters, "-");

Output:

T-h-h-e-D-V-c-e-m-t

Run it here: http://sandbox.onlinephpfunctions.com/code/45c72df556c19e239e09ba0e334ace1e40ce809c

Here's an interesting reference thread for PHP fibonacci: PHP Fibonacci Sequence

Edit: this code will return an undefined index because it doesn't stop when it exceeds strlen

Made a similar edit as @mkaatman

Here's my 3v4l: https://3v4l.org/5KIZD

<?php
$message = 'The Da Vinci Code is a 2003 mystery-detective novel by Dan Brown';
$message = str_replace(' ', '', $message);
$str_split = str_split($message);

$x = 0;    
$y = 1;
$farray[0] = 0;
$farray[1] = 1;

for($i=2;$i<=10;$i++)    
{    
    $z = $x + $y;    
    $farray[] = $z;     
    $x=$y;       
    $y=$z;     
}  

foreach($farray as $key=>$fvalue){
       //echo $key . " => " . $fvalue . "
";
       echo $str_split[$fvalue]."-";
}

?>

Your code was almost correct.

You would delete spaces and add two start element of array.

    $message   = str_replace( ' ', '', 'The Da Vinci Code is a 2003 mystery-detective novel by Dan Brown' );
    $str_split = str_split( $message );

    $x = 0;
    $y = 1;

    $farray[] = $x;
    $farray[] = $y;

    for ( $i = 2; $i <= 10; $i ++ ) {
        $z        = $x + $y;
        $farray[] = $z;
        $x        = $y;
        $y        = $z;
    }

    foreach ( $farray as $key => $fvalue ) {
        echo $str_split[ $fvalue ] . "-";
    }

The shortest possible way would be just using the preg_replace and echo string position;

<?php
$message = 'The Da Vinci Code is a 2003 mystery-detective novel by Dan     Brown';
$message = preg_replace('/\s+/', '', $message);
$strlenggth = strlen($message);
echo $message[$x]; 
$x = 0;    
$y = 1;

for($i=0;$x<=$strlenggth;$i++)    
{    
$z = $x + $y;    
$farray[] = $z;     
$x=$y;       
$y=$z; 
echo "-".$message[$x]; 
}
?>

Alternatively, you can use recursion to avoid using too many for loops:

$message = str_replace(" ", "", $message);

for($i = 0; $i < 10; $i++) {
    echo substr($message, getNthValue($i), 1);
    if($i != 9) {
        echo "-";
    }
}

function getNthValue($n) {
    if($n <= 1) {
        return $n;
    }
    if($n > 1) {
        return getNthValue($n-1) + getNthValue($n-2);
    }
}

Recursive version and wrapped in a function, string length should be irrilevant:

function fiboSecret($msg, $num = 0, $fib = [], $secret = []){
  if(count($fib) > 0) {
    if($num == 1){
      if(strlen($msg) > 1){
        $secret[] = strtoupper($msg[1]);
        return fiboSecret($msg, ++$num, array(0, 1), $secret); 
      }else{
        return $secret;
      }
    }
    $lastFibo = $fib[count($fib) - 1];
    if(array_key_exists($lastFibo, str_split($msg))){
      $secret[] = strtoupper($msg[$lastFibo]);
      $fib[] = $fib[$num-1] + $fib[$num-2];
      return fiboSecret($msg, ++$num, $fib, $secret);
    }else{
      return $secret;
    }
  }else if(strlen($msg) > 0){
    $msg = preg_replace('/\s+/', '', $msg);
    $secret[] = strtoupper($msg[0]);
    return fiboSecret($msg, ++$num, array(0), $secret); 
  }else{
    return [];
  }
  if(array_key_exists($lastFibo, str_split($msg))){
    $secret[] = strtoupper($msg[$lastFibo]);
    $num++;
  }
}