Recognize that variable vars are not assignments

VariableAnalysis/Sniffs/CodeAnalysis/VariableAnalysisSniff.php

@@ -517,6 +517,12 @@ protected function checkForAssignment(File $phpcsFile, $stackPtr, $varName, $cur
       return false;
     }
 
+    // Is this a variable variable? If so, it's not an assignment to the current variable.
+    if ($this->checkForVariableVariable($phpcsFile, $stackPtr, $varName, $currScope)) {
+      Helpers::debug('found variable variable');
+      return false;
+    }
+
     // Plain ol' assignment. Simpl(ish).
     $writtenPtr = Helpers::findWhereAssignExecuted($phpcsFile, $assignPtr);
     if ($writtenPtr === false) {
@@ -526,6 +532,22 @@ protected function checkForAssignment(File $phpcsFile, $stackPtr, $varName, $cur
     return true;
   }
 
+  protected function checkForVariableVariable(File $phpcsFile, $stackPtr, $varName, $currScope) {
+    $tokens = $phpcsFile->getTokens();
+    $token = $tokens[$stackPtr];
+
+    if (!isset($tokens[$stackPtr - 1]['code'])) {
+      return false;
+    }
+    if ($tokens[$stackPtr - 1]['code'] === T_DOLLAR) {
+      return true;
+    }
+    if ($tokens[$stackPtr - 1]['code'] === T_OPEN_CURLY_BRACKET && isset($tokens[$stackPtr - 2]['code']) && $tokens[$stackPtr - 2]['code'] === T_DOLLAR) {
+      return true;
+    }
+    return false;
+  }
+
   protected function checkForListShorthandAssignment(File $phpcsFile, $stackPtr, $varName, $currScope) {
     $tokens = $phpcsFile->getTokens();
     $token  = $tokens[$stackPtr];

VariableAnalysis/Tests/CodeAnalysis/VariableAnalysisTest.php

@@ -540,6 +540,30 @@ public function testVariableFunctionCallsCountAsUsage() {
     $this->assertEquals($expectedErrors, $lines);
   }
 
+  public function testVariableVariables() {
+    $fixtureFile = $this->getFixture('VariableVariablesFixture.php');
+    $phpcsFile = $this->prepareLocalFileForSniffs($this->getSniffFiles(), $fixtureFile);
+    $phpcsFile->process();
+    $lines = $this->getWarningLineNumbersFromFile($phpcsFile);
+    $expectedWarnings = [
+      4,
+      10,
+      16,
+      22,
+      29,
+      35,
+      41,
+      47,
+      48,
+      52,
+      53,
+    ];
+    $this->assertEquals($expectedWarnings, $lines);
+    $lines = $this->getErrorLineNumbersFromFile($phpcsFile);
+    $expectedErrors = [];
+    $this->assertEquals($expectedErrors, $lines);
+  }
+
   public function testTraitsAllowPropertyDefinitions() {
     $fixtureFile = $this->getFixture('TraitWithPropertiesFixture.php');
     $phpcsFile = $this->prepareLocalFileForSniffs($this->getSniffFiles(), $fixtureFile);

VariableAnalysis/Tests/CodeAnalysis/fixtures/VariableVariablesFixture.php

@@ -0,0 +1,56 @@
+<?php
+
+function usedVariableVariableInReturn() {
+    $foo = true; // this is used but it requires execution to know that
+    $varName = 'foo';
+    return $$varName;
+}
+
+function usedVariableVariableInEcho() {
+    $foo = true; // this is used but it requires execution to know that
+    $varName = 'foo';
+    echo $$varName;
+}
+
+function usedVariableVariableInLeftAssignment() {
+    $foo = true; // the below is assignment, not a read, so this should be a warning
+    $marName = 'foo';
+    $$marName = false;
+}
+
+function usedVariableVariableInRightAssignment() {
+    $foo = true; // this is used but it requires execution to know that
+    $varName = 'foo';
+    $bar = $$varName;
+    echo $bar;
+}
+
+function usedVariableVariableInEchoWithBraces() {
+    $foo = true; // this is used but it requires execution to know that
+    $varName = 'foo';
+    echo ${$varName};
+}
+
+function usedVariableVariableInLeftAssignmentWithBraces() {
+    $foo = true; // this is used but it requires execution to know that
+    $varName = 'foo';
+    ${$varName} = false;
+}
+
+function usedVariableVariableInEchoWithBracesInString() {
+    $foo = true; // this is used but it requires execution to know that
+    $varName = 'foo';
+    echo "${$varName}";
+}
+
+function undefinedVariableVariableInEcho() {
+    $foo = true; // this should be a warning
+    echo $$varName; // this should be a warning
+}
+
+function usedVariableVariableTwoLevels() {
+    $foo = 'hello'; // this is used but it requires execution to know that
+    $varNameOne = 'foo'; // this is used but it requires execution to know that
+    $varNameTwo = 'varNameOne';
+    return $$$varNameTwo;
+}