Thanks a lot Vishal, it got saved finally but when running test class, it is showing this Fatal Error as
'Class.PhaseDeleteAllLinesExt.deleteAllLines: line 52, column 1'
'Class.PhaseDeleteAllLinesExtTest.MyTest2: line 61, column 1'
'System.NullPointerException: Attempt to de-reference a null object'

and at line 52 we have class code as

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public class PhaseDeleteAllLinesExt
{
    public Project_Phase_Plan__c pplan{get;set;}
    public String deleteWarning{get;set;}
    public Boolean deleteOK{get;set;}
    public Boolean showCancel{get;set;}
    public Boolean showClose{get;set;}
    public Set<String> deleteAllOK  = new Set<String> {'Draft', 'Pre-Release Review', 'Confirmation Pending', 'Confirmed'};
    public PageReference currentPage = ApexPages.currentPage();
    
    public PhaseDeleteAllLinesExt(ApexPages.StandardSetController stdSetController)
    {
        //
        // Get the Order Release Information
        //
        deleteOK = false;
        showCancel = true;
        showClose = false;
        Id id = ApexPages.CurrentPage().getParameters().get('id');
        if (id != null)
        {
            System.debug('OER ID = ' + String.valueOf(id));
            pplan = [SELECT Id, Name FROM Project_Phase_Plan__c WHERE Id = :id LIMIT 1];
        }
        if (pplan == null)
        {
            ApexPages.Message noOEError = new ApexPages.Message(ApexPages.Severity.ERROR,'** ERROR - Unable to retrieve the Project Phase Plan Information');
            ApexPages.addMessage(noOEError);
            showClose = true;
            showCancel = false;         
        }
        else
        {
            
                deleteWarning = 'WARNING: You have requested the deletion of all Project Phase for Project Phase Plan ' + pplan.Name  + '<br>'+
                'Click the Continue button to confirm deletion; click the Cancel button to return to the OProject Phase Plan Without deleting the Lines';
                deleteOK = true;
          
        }
    
    }
    
    public PageReference close()
    {
       return new PageReference('javascript:window.self.close()');
    }
    
    public PageReference deleteAllLines()
    {
        List<Database.DeleteResult> drList = new List<Database.DeleteResult>();
        Boolean hasErrors = false;


        List<Project_Phase__c> linesToDelete = [SELECT Id, Project_Phase_Plan__c FROM Project_Phase__c WHERE Project_Phase_Plan__c = :pplan.Id];                                  //this is LINE 52 ERROR
        


        if (linesToDelete.size() > 0)
        {
            ApexPages.Message databaseErrorMsg;
            drList = Database.delete(linesToDelete, true);
            Integer i = 0;      

            for(Database.DeleteResult dr : drList)
            {
                if (!dr.isSuccess())
                {
                    hasErrors = true;
                    for(Database.Error err : dr.getErrors())
                    {
                        databaseErrorMsg = new ApexPages.Message(ApexPages.Severity.ERROR, '*** An error occurred deleting Project Phase ID ' +
                            String.valueOf(linesToDelete[i].id) + '; ' + err.getMessage());
                        ApexPages.addMessage(databaseErrorMsg);
                    }
                }
                i++;
            }
        }
        if (hasErrors)
        {
            deleteWarning = 'Errors occurred while trying to delete the Project Phases; click the Cancel button to return to the Order Entry Release Page';
        }
        else
        {
            deleteWarning = String.valueOf(linesToDelete.size()) + ' Project Phase were successfully deleted.<br>' +
                'Close this window and then refresh the Project Phase Plan page to see the results';
        }
        deleteOK = false;
        showClose = true;
        showCancel = false;
        
        

        return currentPage;
 
        
    }
}

***************

and at line 61 code is from test class and below is the code

************
@isTest
public class PhaseDeleteAllLinesExtTest {
        
        static testMethod void MyTest()
        {       

//            list<opportunity> opp = [select id, name from opportunity where id='0060h000010bfWz'];
            
            Project_Phase_Plan__c  plan = new Project_Phase_Plan__c ();
            plan.Phase_Plan_Description__c = 'Testing';
            //add other fields here

            insert plan;
            
            Project_Phase__c phase = new Project_Phase__c();
            phase.Project_Phase_Plan__c = plan.Id;
            phase.Project_Phase__c = 1;
            phase.Trade__c = 'Glazier';
            //add other fields here
            
            insert phase ;
            
            list<Project_Phase_Plan__c> newplan = new list<Project_Phase_Plan__c>([select Id, name, Phase_Plan_Description__c from Project_Phase_Plan__c]);
            PageReference pageRef = Page.ProjectPhaseDeleteAllLines;

            pageRef.getparameters().put('Id', plan.Id);
            Test.setCurrentPage(pageRef);
            ApexPages.StandardSetController sc = new ApexPages.StandardSetController(newplan);

            PhaseDeleteAllLinesExt ext = new PhaseDeleteAllLinesExt(sc);
            ext.deleteAllLines();

           ext.close();

        }
    
         static testMethod void MyTest2()
             {
                 Project_Phase_Plan__c  plan = new Project_Phase_Plan__c ();                
                 plan.Phase_Plan_Description__c = 'Testing';
                //plan.Name ='Test';
                //add other fields

                insert plan;

                Project_Phase__c phase = new Project_Phase__c();
                phase.Project_Phase_Plan__c = plan.Id;
                phase.Project_Phase__c = 1;
                phase.Trade__c = 'Glazier';
                 //phase.Name = 'Test' ;
                //add other fields

                insert phase ;

                list<Project_Phase_Plan__c> newplan = new list<Project_Phase_Plan__c>([select Id, name, Phase_Plan_Description__c from Project_Phase_Plan__c]);
                PageReference pageRef = Page.ProjectPhaseDeleteAllLines;
                Test.setCurrentPage(pageRef);
                ApexPages.StandardSetController sc = new ApexPages.StandardSetController(newplan);

            PhaseDeleteAllLinesExt ext = new PhaseDeleteAllLinesExt(sc);
            ext.deleteAllLines();     

       ext.close();

        }
    }

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