diff --git a/phpseclib/Crypt/Salsa20.php b/phpseclib/Crypt/Salsa20.php
index 526027f30..a7949a1db 100644
--- a/phpseclib/Crypt/Salsa20.php
+++ b/phpseclib/Crypt/Salsa20.php
@@ -316,7 +316,7 @@ private function crypt(string $text, int $mode): string
             foreach ($blocks as &$block) {
                 $block ^= static::salsa20($this->p1 . pack('V', $i++) . $this->p2);
             }
-
+            unset($block);
             return implode('', $blocks);
         }
 
@@ -356,6 +356,7 @@ private function crypt(string $text, int $mode): string
                     foreach ($blocks as &$block) {
                         $block ^= static::salsa20($this->p1 . pack('V', $buffer['counter']++) . $this->p2);
                     }
+                    unset($block);
                 }
                 $encrypted = implode('', $blocks);
                 $temp = static::salsa20($this->p1 . pack('V', $buffer['counter']++) . $this->p2);
@@ -378,6 +379,7 @@ private function crypt(string $text, int $mode): string
                 foreach ($blocks as &$block) {
                     $block ^= static::salsa20($this->p1 . pack('V', $buffer['counter']++) . $this->p2);
                 }
+                unset($block);
                 $ciphertext .= implode('', $blocks);
             }
         }