GQL-47: Completed implementation and documentation of the query variables feature:

- Modified the behavior of QueryBuilder::setVariable to not receive a Variable object
- Added documentation and examples to the readme.md

Author: mghoneimy

README.md

@@ -111,6 +111,34 @@ The RawObject class being constructed is used for injecting the string into the
 into the RawObject constructor will be put in the query as it is without any custom formatting normally done by the
 query class.
 
+## Query With Variables
+```
+$gql = (new Query('companies'))
+    ->setVariables(
+        [
+            new Variable('name', 'String', true),
+            new Variable('limit', 'Int', false, 5)
+        ]
+    )
+    ->setArguments(['name' => '$name', 'first' => '$limit'])
+    ->setSelectionSet(
+        [
+            'name',
+            'serialNumber'
+        ]
+    );
+```
+This query shows how variables can be used in this package to allow for dynamic requests enabled by GraphQL standards.
+
+### The Variable Class
+The Variable class is an immutable class that represents a variable in GraphQL standards. Its constructor receives 4
+arguments:
+- name: Represents the variable name
+- type: Represents the variable type according to the GraphQL server schema
+- isRequired (Optional): Represents if the variable is required or not, it's false by default
+- defaultValue (Optional): Represents the default value to be assigned to the variable. The default value will only be
+considered if the isRequired argument is set to false.
+
 # The Query Builder
 The QueryBuilder class can be used to construct Query objects dynamically, which can be useful in some cases. It works
 very similarly to the Query class, but the Query building is divided into steps.
@@ -119,7 +147,8 @@ That's how the "Query With Input Object Argument" example can be created using t
 QueryBuilder:
 ```
 $builder = (new QueryBuilder('companies'))
-    ->setArgument('filter', new RawObject('{name_starts_with: "Face"}'))
+    ->setVariable('namePrefix', 'String', true)
+    ->setArgument('filter', new RawObject('{name_starts_with: $namePrefix}'))
     ->selectField('name')
     ->selectField('serialNumber');
 $gql = $builder->getQuery();
@@ -139,7 +168,7 @@ $client = new Client(
 ```
 
 # Running Queries
-
+## Result Formatting
 Running query with the GraphQL client and getting the results in object structure:
 ```
 $results = $client->runQuery($gql);
@@ -151,6 +180,28 @@ $results = $client->runQuery($gql, true);
 $results->getData()['Company'][1]['branches']['address']
 ```
 
+## Passing Variables to Queries
+Running queries containing variables requires passing an associative array which maps variable names (keys) to variable
+values (values) to the `runQuery` method. Here's an example:
+```
+$gql = (new Query('companies'))
+    ->setVariables(
+        [
+            new Variable('name', 'String', true),
+            new Variable('limit', 'Int', false, 5)
+        ]
+    )
+    ->setArguments(['name' => '$name', 'first' => '$limit'])
+    ->setSelectionSet(
+        [
+            'name',
+            'serialNumber'
+        ]
+    );
+$variablesArray = ['name' => 'Tech Co.', 'first' => 5];
+$results = $client->runQuery($gql, true, $variablesArray);
+```
+
 # Mutations
 Mutations follow the same rules of queries in GraphQL, they select fields on returned objects, receive arguments, and
 can have sub-fields.

src/QueryBuilder/AbstractQueryBuilder.php

@@ -100,13 +100,16 @@ protected function setArgument(string $argumentName, $argumentValue)
     }
 
     /**
-     * @param Variable $variable
+     * @param string $name
+     * @param string $type
+     * @param bool   $isRequired
+     * @param null   $defaultValue
      *
      * @return $this
      */
-    protected function setVariable(Variable $variable)
+    protected function setVariable(string $name, string $type, bool $isRequired = false, $defaultValue = null)
     {
-        $this->variables[] = $variable;
+        $this->variables[] = new Variable($name, $type, $isRequired, $defaultValue);
 
         return $this;
     }

src/QueryBuilder/QueryBuilder.php

@@ -40,12 +40,15 @@ public function setArgument(string $argumentName, $argumentValue)
     /**
      * Changing method visibility to public
      *
-     * @param Variable $variable
+     * @param string $name
+     * @param string $type
+     * @param bool   $isRequired
+     * @param null   $defaultValue
      *
      * @return AbstractQueryBuilder|QueryBuilder
      */
-    public function setVariable(Variable $variable)
+    public function setVariable(string $name, string $type, bool $isRequired = false, $defaultValue = null)
     {
-        return parent::setVariable($variable);
+        return parent::setVariable($name, $type, $isRequired, $defaultValue);
     }
 }

tests/QueryBuilderTest.php

@@ -68,8 +68,8 @@ public function testEmptySelectionSet()
     public function testAddVariables()
     {
         $this->queryBuilder
-            ->setVariable(new Variable('var', 'String'))
-            ->setVariable(new Variable('intVar', 'Int', false, 4))
+            ->setVariable('var', 'String')
+            ->setVariable('intVar', 'Int', false, 4)
             ->selectField('fieldOne');
         $this->assertEquals(
             'query($var: String $intVar: Int=4) {
@@ -87,11 +87,11 @@ public function testAddVariables()
     public function testAddVariablesToSecondLevelQueryDoesNothing()
     {
         $this->queryBuilder
-            ->setVariable(new Variable('var', 'String'))
+            ->setVariable('var', 'String')
             ->selectField('fieldOne')
             ->selectField(
                 (new QueryBuilder('Nested'))
-                    ->setVariable(new Variable('var', 'String'))
+                    ->setVariable('var', 'String')
                     ->selectField('fieldTwo')
             );
         $this->assertEquals(