MD5 bypasses the first formula: weak comparison bypass

<?php

//Try to output the hash value to the terminal in binary format
var_dump(md5('Hello World', true));

//Try to convert the binary representation of the hash value to hexadecimal and output it to the terminal
var_dump(bin2hex(md5('Hello World', true)));

Execution effect

Since the result of converting binary data into text contains the invisible character newline character, the output result of var_dump(md5('Hello World', true)); Presented as two lines.

string(16) "?
d?uA."
string(32) "b10a8db164e0754105b7a99be72e3fe5"

Implicit type conversion

In PHP, Implicit Type Conversion means that in certain operations, PHP will automatically convert the data from one data type to Another data type without writing explicit code for the type conversion.

PHP's implicit type conversion will convert the operands according to certain rules (specific analysis of specific situations), so that the relevant operations can can proceed normally.

String concatenation

When performing a string concatenation operation using the period operator ., PHP will attempt to convert other data types to the string data type. For this, please refer to the following example:

<?php

//Try to concatenate two strings
var_dump('Hello ' . 'World');

// Try to concatenate a value with a string
var_dump('1 + 1 = ' . 2);

//Try to concatenate two values
var_dump(1 . 1);

Execution effect

string(11) "Hello World"
string(9) "1 + 1 = 2"
string(2) "11"

Mathematical operations

When performing mathematical operations via mathematical operators, PHP will attempt to convert data of other data types to numeric types. For this, please refer to the following example:

<?php

// Attempt to subtract the Boolean value true from the value 1
var_dump(true - 1);

// Attempt to add the boolean values true and false
var_dump(true + false);

// Attempt to perform multiplication between strings
var_dump('2' * '150');

// The string 100djdj will be converted to 100
var_dump('100djdj' / 10);

// The string djdj100 will be converted to zero
var_dump('djdj100' / 10);

Execution effect

int(0)
int(1)
int(300)
int(10)
int(0)

Boolean judgment

Where a Boolean value is required, PHP will attempt to convert non-Boolean data to Boolean data. For this, please refer to the following example:

<?php

//Try to convert empty string to boolean
if(''){<!-- -->
    print('Hello World' . "\\
");
}

//Try to convert the string Hello World to a boolean
if('Hello World'){<!-- -->
    print('Hello China' . "\\
");
}

//Try to convert the value 999 to a boolean
if(999){<!-- -->
    print('Jiujiujiu' . "\\
");
}

Execution effect

Hello China
jiujiujiu

Equality operator

There are two equality operators in PHP, namely the weak type equality operator == and the strong type equality operator ===, both of which can be used to judge two The operands are equal, but there are some differences.

The difference between the two is that the weakly typed equality operator will automatically perform type conversion to use Both belong to the same data type. When comparing, the strongly typed equality operator requires that the type and value of the two values must be identical >, No type conversion. For this, please refer to the following example:

<?php

// Use weak type comparison operators to compare values and strings
// When comparing rows, PHP first converts strings to numeric values.

// Since the two values are the same after being converted to the same type,
// So it will return true.
var_dump('123' == 123);

// Since the data types and values of the two are different, so
// Will return false.
var_dump('123' === 123);

Execution effect

bool(true)
bool(false)

Scientific notation

Scientific notation

In PHP, e and E both represent Scientific Notation. Scientific notation consists of two parts: base and exponent. Commonly used to represent very large or very small values.

In scientific notation, the base usually is a floating-point number between 1 and 10, while the exponent is an integer representing the desired Multiply the base by the power of 10. The base and exponent are separated by the characters e or E.

Give me a chestnut

<?php

// 3.78 * 10 ^ 3
var_dump(3.78e3);

// 3 * 10 ^ -1
var_dump(3E-1);

Execution effect

float(3780)
float(0.3)

Prefix 0E and 0e

Zero raised to any power is zero, so any number expressed in scientific notation prefixed with 0E or 0e will result in The value is zero. For this, please refer to the following example:

<?php

var_dump(0e3280);
var_dump((float)'0e30284083');
var_dump((float)'0esjlfjsld');

Execution effect

float(0)
float(0)
float(0)

Specific rules for converting strings into numerical values in PHP8 and other versions of PHP

PHP8

Numerical string

Numeric string refers to a string containing numeric characters. Numeric strings can be used to directly represent a numerical value.

Give me a chestnut

"123"
"-42"
" + 384"

"3.14"
"-0.5"
"0.0000"
"00000000.0000"

"2.5e3"
"1.2e-2"
" + 42.0E0"

"0004746"
"0305940"
" 484748 "
" 4847 "
"3847"

Note:

1. Strings containing symbols that represent values in other bases (non-decimal) cannot be called numeric strings (These symbols are not usually used in mathematics to identify values in other bases), such as 0x1F, 0b10101 and other strings. In PHP, octal values are represented by leading zeros, but in numeric strings, leading zeros are considered normal Numbers do not have the function of identifying octal values.
2. If there is a need to convert strings in other bases (non-decimal) into numerical values, you can consider using functions such as intval() for explicit type conversion.
3. A string containing whitespace characters such as spaces Why can it also be a numerical string? You can understand it as What appears to be a numerical value at first glance is a numerical string.

Optimization