Fix internal variable bleeding in REPL eval context (#80)

Co-authored-by: swissspidy <841956+swissspidy@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Pascal Birchler <pascal.birchler@gmail.com>

Author: Copilot

features/shell.feature

@@ -78,6 +78,44 @@ Feature: WordPress REPL
       history: -1: invalid option
       """
 
+  Scenario: User can define variable named $line
+    Given a WP install
+    And a session file:
+      """
+      $line = 'this should work';
+      $line;
+      """
+
+    When I run `wp shell --basic < session`
+    Then STDOUT should contain:
+      """
+      => string(16) "this should work"
+      """
+    And STDOUT should contain:
+      """
+      => string(16) "this should work"
+      """
+
+  Scenario: User can define variables named $out and $evl
+    Given a WP install
+    And a session file:
+      """
+      $out = 'out should work';
+      $evl = 'evl should work';
+      $out;
+      $evl;
+      """
+
+    When I run `wp shell --basic < session`
+    Then STDOUT should contain:
+      """
+      => string(15) "out should work"
+      """
+    And STDOUT should contain:
+      """
+      => string(15) "evl should work"
+      """
+
   Scenario: Shell with hook parameter
     Given a WP install
     And a session file:

src/WP_CLI/Shell/REPL.php

@@ -19,38 +19,38 @@ public function __construct( $prompt ) {
 	public function start() {
 		// @phpstan-ignore while.alwaysTrue
 		while ( true ) {
-			$line = $this->prompt();
+			$__repl_input_line = $this->prompt();
 
-			if ( '' === $line ) {
+			if ( '' === $__repl_input_line ) {
 				continue;
 			}
 
-			$line = rtrim( $line, ';' ) . ';';
+			$__repl_input_line = rtrim( $__repl_input_line, ';' ) . ';';
 
-			if ( self::starts_with( self::non_expressions(), $line ) ) {
+			if ( self::starts_with( self::non_expressions(), $__repl_input_line ) ) {
 				ob_start();
 				// phpcs:ignore Squiz.PHP.Eval.Discouraged -- This is meant to be a REPL, no way to avoid eval.
-				eval( $line );
-				$out = (string) ob_get_clean();
-				if ( 0 < strlen( $out ) ) {
-					$out = rtrim( $out, "\n" ) . "\n";
+				eval( $__repl_input_line );
+				$__repl_output = (string) ob_get_clean();
+				if ( 0 < strlen( $__repl_output ) ) {
+					$__repl_output = rtrim( $__repl_output, "\n" ) . "\n";
 				}
-				fwrite( STDOUT, $out );
+				fwrite( STDOUT, $__repl_output );
 			} else {
-				if ( ! self::starts_with( 'return', $line ) ) {
-					$line = 'return ' . $line;
+				if ( ! self::starts_with( 'return', $__repl_input_line ) ) {
+					$__repl_input_line = 'return ' . $__repl_input_line;
 				}
 
 				// Write directly to STDOUT, to sidestep any output buffers created by plugins
 				ob_start();
 				// phpcs:ignore Squiz.PHP.Eval.Discouraged -- This is meant to be a REPL, no way to avoid eval.
-				$evl = eval( $line );
-				$out = (string) ob_get_clean();
-				if ( 0 < strlen( $out ) ) {
-					echo rtrim( $out, "\n" ) . "\n";
+				$__repl_eval_result = eval( $__repl_input_line );
+				$__repl_output      = (string) ob_get_clean();
+				if ( 0 < strlen( $__repl_output ) ) {
+					echo rtrim( $__repl_output, "\n" ) . "\n";
 				}
 				echo '=> ';
-				var_dump( $evl );
+				var_dump( $__repl_eval_result );
 				fwrite( STDOUT, (string) ob_get_clean() );
 			}
 		}