If value of first array is found on second array, replace and echo it

You can do

$array1 = array('first', 'second', 'third', 'fourth');
$array2 = array('second' => 'bye', 'first' => 'hello',  'third' => 'see you', 'fifth' => 'good evening');
print_r(array_intersect_key( $array2 ,array_flip($array1)));

Output

Array
(
    [second] => bye
    [first] => hello
    [third] => see you
)

Sandbox

array_intersect_key() returns an array containing all the entries of array1 which have keys that are present in all the arguments. https://www.php.net/manual/en/function.array-intersect-key.php

And

array_flip() returns an array in flip order, i.e. keys from array become values and values from array become keys. https://www.php.net/manual/en/function.array-flip.php

Update

I know but it's array1 that is the "string". Array2 is the replacements. See the question: I want to echo hello bye see you fourth

$array1 = array('first', 'second', 'third', 'fourth');
$array2 = array('second' => 'bye', 'first' => 'hello',  'third' => 'see you', 'fifth' => 'good evening');

echo implode(' ', array_intersect_key( array_merge(array_combine($array1,$array1), $array2) ,array_flip($array1)));

Output

hello bye see you fourth

Sandbox

The difference with this one is this bit

 $array2 = array_merge(array_combine($array1,$array1), $array2)

What this does is make a new array with array_combine which has the same keys as values. In this case:

array('first'=>'first', 'second'=>'second', 'third'=>'third',  'fourth'=>'fourth')

Then we use the fact that when merging 2 rows the second array will overwrite keys from the first so we merge that with our original $array2

$array2 = array('second' => 'bye', 'first' => 'hello',  'third' => 'see you', 'fourth'=>'fourth', 'fifth' => 'good evening');

So by doing this "extra" step we insure that all elements in $array1 exist in $array2 - the order of $array1 is also preserved - in this case it just adds 'fourth'=>'fourth' but that is exactly what we need for the intersect to work.

Then we just implode it.

UPDATE2

This is a simpler one using preg_filter and preg_replace

$array1 = array('first', 'second', 'third',  'fourth');
$array2 = array('second' => 'bye', 'first' => 'hello',  'third' => 'see you', 'fifth' => 'good evening');

echo implode(' ',preg_replace(preg_filter('/(.+)/', '/(\1)/i', array_keys($array2)), $array2, $array1));

Output

hello bye see you fourth

It's very similar to the str_replace one, so I didn't want to post it as it's own answer. But it does have a few benefits over str_replace. For example

echo implode(' ',preg_replace(preg_filter('/(.+)/', '/\b(\1)\b/i', array_keys($array2)), $array2, $array1));

Now we have \b word boundaries, which means first wont match firstone etc.

Sorry I really enjoy doing things like this, it's "fun" for me.

You can access $array2[$key] and use the null-coalescing operator since you apparently want to fall back to the value from $array1 if it doesn't exist as a key in $array2.

foreach ($array1 as $key) {
  echo $array2[$key] ?? $key, ' ';
  // [PHP5] echo isset($array2[$key]) ? $array2[$key] : $key, ' ';
}

Demo: https://3v4l.org/3vQYt

Note: this adds a potentially unwanted space at the end. If that's the case, either add a condition to the loop or do something like this instead:

echo implode(' ', array_map(function ($key) use ($array2) { 
  return $array2[$key] ?? $key;
  // [PHP5] return isset($array2[$key]) ? $array2[$key] : $key;
}, $array1));

Demo: https://3v4l.org/VCDVt

foreach ($array1 as $extra) {
      foreach ($array2 as $key=>$value) {
          if($extra==$key){
               echo $value;
          }
      }
}

something like this

Just use isset:

foreach($array1 as $e) {
    $val = isset($array2[$e]) ? $array2[$e] : $e;
    echo $val;
}

Doc can be found here: https://www.php.net/manual/en/function.isset.php

You can just implode the first array then use str_replace to replace the words from the second in the imploded string.
No need for loops in my opinion.

echo str_replace(array_keys($array2), $array2, implode(" ", $array1));
//hello bye see you fourth

Array_keys will take the keys from array2 and make them the find, then the values from array2 will be the replacements.

https://3v4l.org/UIWV0