Data Type Automatic Conversion

This section provides rules on how operands are converted to data types required by the operation. PHP always finds a way to convert data from any type to any other type.

When the PHP evaluating an operation, it will automatically convert the data type of an operand if it does not match the data type required by the operator. Here are some basic data type automatic conversion rules:

To show you some of these rules, I wrote the following sample PHP script, TypeAutoConversion.php:

<?php
#  TypeAutoConversion.php
#- Copyright 2003 (c) HerongYang.com. All Rights Reserved.
#
   print "\n Logical operations:\n";
   print "    NULL==FALSE: "; var_dump(NULL==FALSE);
   print "    0==FALSE: "; var_dump(0==FALSE);
   print "    \"\"==FALSE: "; var_dump(""==FALSE);
   print "    \"0\"==FALSE: "; var_dump("0"==FALSE);
   print "    \"0 - zero\"==TRUE: "; var_dump("0 - zero"==TRUE);
   print "    \"zero\"==TRUE: "; var_dump("zero"==TRUE);

   print "\n Logical operations:\n";
   print "    3.14 * \"2.0\": "; var_dump(3.14 * "2.0");
   print "    3.14 * \"2.0%\": "; var_dump(3.14 * "2.0%");
   print "    3.14 * \"\$2.0\": "; var_dump(3.14 * "$2.0");
   print "    3.14 * TRUE: "; var_dump(3.14 * TRUE);
   print "    3.14 * FALSE: "; var_dump(3.14 * FALSE);

   print "\n Conparison operations:\n";
   print "    3.14 < \"Hello\": "; var_dump(3.14 < "Hello");
   print "    3.14 > \"Hello\": "; var_dump(3.14 > "Hello");
   print "    \"3.14\" > \"Hello\": "; var_dump("3.14" > "Hello");

   print "\n String operations:\n";
   print "    \"Hello \" . TRUE: "; var_dump("Hello " . TRUE);
   print "    \"Hello \" . FALSE: "; var_dump("Hello " . FALSE);
   print "    \"Hello \" . NULL: "; var_dump("Hello " . NULL);
   print "    \"Hello \" . 1.0/3.0: "; var_dump("Hello " . 1.0/3.0);
?>

If you run this sample script in PHP 7 and higher, you should get output with some messages. This is because now the default message level is set to E_NOTICE. You can change the setting or modify the code to make sure all variables are defined before using them.

herong> \php-7.3\php TypeAutoConversion.php

 Logical operations:
    NULL==FALSE: bool(true)
    0==FALSE: bool(true)
    ""==FALSE: bool(true)
    "0"==FALSE: bool(true)
    "0 - zero"==TRUE: bool(true)
    "zero"==TRUE: bool(true)

 Logical operations:
    3.14 * "2.0": float(6.28)
    3.14 * "2.0%": PHP Notice:  A non well formed numeric value
       encountered in C:\herong\TypeAutoConversion.php on line 14

Notice: A non well formed numeric value encountered in
   C:\herong\TypeAutoConversion.php on line 14
float(6.28)
    3.14 * "$2.0": PHP Warning:  A non-numeric value encountered in
       C:\herong\TypeAutoConversion.php on line 15

Warning: A non-numeric value encountered in
   C:\herong\TypeAutoConversion.php on line 15
float(0)
    3.14 * TRUE: float(3.14)
    3.14 * FALSE: float(0)

 Conparison operations:
    3.14 < "Hello": bool(false)
    3.14 > "Hello": bool(true)
    "3.14" > "Hello": bool(false)

 String operations:
    "Hello " . TRUE: string(7) "Hello 1"
    "Hello " . FALSE: string(6) "Hello "
    "Hello " . NULL: string(6) "Hello "
    "Hello " . 1.0/3.0: string(22) "Hello 0.33333333333333"

If you run this sample script in PHP 5 and lower, you should get output without any message:

herong> \php5.6\php TypeAutoConversion.php

 Logical operations:
    NULL==FALSE: bool(true)
    0==FALSE: bool(true)
    ""==FALSE: bool(true)
    "0"==FALSE: bool(true)
    "0 - zero"==TRUE: bool(true)
    "zero"==TRUE: bool(true)

 Logical operations:
    3.14 * "2.0": float(6.28)
    3.14 * "2.0%": float(6.28)
    3.14 * "$2.0": float(0)
    3.14 * TRUE: float(3.14)
    3.14 * FALSE: float(0)

 Conparison operations:
    3.14 < "Hello": bool(false)
    3.14 > "Hello": bool(true)
    "3.14" > "Hello": bool(false)

 String operations:
    "Hello " . TRUE: string(7) "Hello 1"
    "Hello " . FALSE: string(6) "Hello "
    "Hello " . NULL: string(6) "Hello "
    "Hello " . 1.0/3.0: string(20) "Hello 0.333333333333"

Table of Contents

 About This Book

 Introduction and Installation of PHP

 PHP Script File Syntax

 PHP Data Types and Data Literals

 Variables, References, and Constants

Expressions, Operations and Type Conversions

 What Is an Expression

 What Is an Operation

 Precedence of Operations

Data Type Automatic Conversion

 Conditional Statements - "if" and "switch"

 Loop Statements - "while", "for", and "do ... while"

 Function Declaration, Arguments, and Return Values

 Arrays - Ordered Maps

 Interface with Operating System

 Introduction of Class and Object

 Integrating PHP with Apache Web Server

 Retrieving Information from HTTP Requests

 Creating and Managing Sessions in PHP Scripts

 Sending and Receiving Cookies in PHP Scripts

 Controlling HTTP Response Header Lines in PHP Scripts

 Managing File Upload

 MySQL Server Connection and Access Functions

 Functions to Manage Directories, Files and Images

 SOAP Extension Function and Calling Web Services

 SOAP Server Functions and Examples

 Localization Overview of Web Applications

 Using Non-ASCII Characters in HTML Documents

 Using Non-ASCII Characters as PHP Script String Literals

 Receiving Non-ASCII Characters from Input Forms

 "mbstring" Extension and Non-ASCII Encoding Management

 Managing Non-ASCII Character Strings with MySQL Servers

 Parsing and Managing HTML Documents

 Configuring and Sending Out Emails

 Image and Picture Processing

 Managing ZIP Archive Files

 Managing PHP Engine and Modules on macOS

 Managing PHP Engine and Modules on CentOS

 Archived Tutorials

 References

 Full Version in PDF/EPUB