Questions about the PHP text box

Problems with PHP text boxes
Just learned PHP, encountered a lot of problems, please kindly people help solve!

1. The text box already has the content value= "content", how to do click button, make the content of this text box empty or replace existing content with other content

2. How to select a value in a drop-down menu the corresponding content will be displayed in the other text box
For example, when you choose 1, the text box displays 2
Select 2 when the text box displays 2

Please master advice, there is code to send a.

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It should all be JavaScript's problem.
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document.getElementById (' Test '). Value = ' text box empty ';//Empty is clear
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function Delsearch (svalue) {//reloads the window if Nav4 resized
var Searchkey=document.getelementbyid ("Searchkey");
if (Searchkey.value==svalue) {
Searchkey.value= ";
}
}
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JS realize it
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。。。 There is a button in the HTML that is designed to be emptied.

HTML code

 
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     Explore  
    
 ... 
   There is a button in the HTML that is designed to be emptied. 
   
    
 
    
 HTML Code 
    
 
     
    
 
    
 
    
 
    
 
    
 2nd forget what the code says,
   
    
 
    
 As long as you send the drop-down box to the value of PHP to the text box, then the text box will appear on the line. 
    
 
    
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 ..... 
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     discusses  
    
 function Delsearch (svalue) {//reloads 
   The window if Nav4 resized 
    
 var Searchkey=document.getelementbyid ("Searchkey"); 
   
    
 if (searchkey.value==svalue) {
    
 searchkey.value= "; 
    
} 
    
} 
    
 
    
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 onclick= "reset ()" 
 