/
comparison.xml
535 lines (502 loc) · 15.3 KB
/
comparison.xml
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
<?xml version="1.0" encoding="utf-8"?>
<sect1 xml:id="language.operators.comparison">
<title>Comparison Operators</title>
<titleabbrev>Comparison</titleabbrev>
<simpara>
Comparison operators, as their name implies, allow you to compare
two values. You may also be interested in viewing
<link linkend="types.comparisons">the type comparison tables</link>,
as they show examples of various type related comparisons.
</simpara>
<table>
<title>Comparison Operators</title>
<tgroup cols="3">
<thead>
<row>
<entry>Example</entry>
<entry>Name</entry>
<entry>Result</entry>
</row>
</thead>
<tbody>
<row>
<entry>$a == $b</entry>
<entry>Equal</entry>
<entry>&true; if <varname>$a</varname> is equal to <varname>$b</varname> after type juggling.</entry>
</row>
<row>
<entry>$a === $b</entry>
<entry>Identical</entry>
<entry>
&true; if <varname>$a</varname> is equal to <varname>$b</varname>, and they are of the same
type.
</entry>
</row>
<row>
<entry>$a != $b</entry>
<entry>Not equal</entry>
<entry>&true; if <varname>$a</varname> is not equal to <varname>$b</varname> after type juggling.</entry>
</row>
<row>
<entry>$a <> $b</entry>
<entry>Not equal</entry>
<entry>&true; if <varname>$a</varname> is not equal to <varname>$b</varname> after type juggling.</entry>
</row>
<row>
<entry>$a !== $b</entry>
<entry>Not identical</entry>
<entry>
&true; if <varname>$a</varname> is not equal to <varname>$b</varname>, or they are not of the same
type.
</entry>
</row>
<row>
<entry>$a < $b</entry>
<entry>Less than</entry>
<entry>&true; if <varname>$a</varname> is strictly less than <varname>$b</varname>.</entry>
</row>
<row>
<entry>$a > $b</entry>
<entry>Greater than</entry>
<entry>&true; if <varname>$a</varname> is strictly greater than <varname>$b</varname>.</entry>
</row>
<row>
<entry>$a <= $b</entry>
<entry>Less than or equal to </entry>
<entry>&true; if <varname>$a</varname> is less than or equal to <varname>$b</varname>.</entry>
</row>
<row>
<entry>$a >= $b</entry>
<entry>Greater than or equal to </entry>
<entry>&true; if <varname>$a</varname> is greater than or equal to <varname>$b</varname>.</entry>
</row>
<row>
<entry>$a <=> $b</entry>
<entry>Spaceship</entry>
<entry>
An <type>int</type> less than, equal to, or greater than zero when
<varname>$a</varname> is less than, equal to, or greater than
<varname>$b</varname>, respectively.
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
If both operands are
<link linkend="language.types.numeric-strings">numeric strings</link>,
or one operand is a number and the other one is a
<link linkend="language.types.numeric-strings">numeric string</link>,
then the comparison is done numerically.
These rules also apply to the
<link linkend="control-structures.switch">switch</link> statement.
The type conversion does not take place when the comparison is
<literal>===</literal> or <literal>!==</literal> as this involves
comparing the type as well as the value.
</para>
<warning>
<para>
Prior to PHP 8.0.0, if a <type>string</type> is compared to a number
or a numeric string then the <type>string</type> was converted to a
number before performing the comparison. This can lead to surprising
results as can be seen with the following example:
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
var_dump(0 == "a");
var_dump("1" == "01");
var_dump("10" == "1e1");
var_dump(100 == "1e2");
switch ("a") {
case 0:
echo "0";
break;
case "a":
echo "a";
break;
}
?>
]]>
</programlisting>
&example.outputs.7;
<screen>
<![CDATA[
bool(true)
bool(true)
bool(true)
bool(true)
0
]]>
</screen>
&example.outputs.8;
<screen>
<![CDATA[
bool(false)
bool(true)
bool(true)
bool(true)
a
]]>
</screen>
</informalexample>
</para>
</warning>
<para>
<informalexample>
<programlisting role="php">
<![CDATA[
<?php
// Integers
echo 1 <=> 1; // 0
echo 1 <=> 2; // -1
echo 2 <=> 1; // 1
// Floats
echo 1.5 <=> 1.5; // 0
echo 1.5 <=> 2.5; // -1
echo 2.5 <=> 1.5; // 1
// Strings
echo "a" <=> "a"; // 0
echo "a" <=> "b"; // -1
echo "b" <=> "a"; // 1
echo "a" <=> "aa"; // -1
echo "zz" <=> "aa"; // 1
// Arrays
echo [] <=> []; // 0
echo [1, 2, 3] <=> [1, 2, 3]; // 0
echo [1, 2, 3] <=> []; // 1
echo [1, 2, 3] <=> [1, 2, 1]; // 1
echo [1, 2, 3] <=> [1, 2, 4]; // -1
// Objects
$a = (object) ["a" => "b"];
$b = (object) ["a" => "b"];
echo $a <=> $b; // 0
$a = (object) ["a" => "b"];
$b = (object) ["a" => "c"];
echo $a <=> $b; // -1
$a = (object) ["a" => "c"];
$b = (object) ["a" => "b"];
echo $a <=> $b; // 1
// not only values are compared; keys must match
$a = (object) ["a" => "b"];
$b = (object) ["b" => "b"];
echo $a <=> $b; // 1
?>
]]>
</programlisting>
</informalexample>
</para>
<para>
For various types, comparison is done according to the following
table (in order).
</para>
<table xml:id="language.operators.comparison.types">
<title>Comparison with Various Types</title>
<tgroup cols="3">
<thead>
<row>
<entry>Type of Operand 1</entry>
<entry>Type of Operand 2</entry>
<entry>Result</entry>
</row>
</thead>
<tbody>
<row>
<entry><type>null</type> or <type>string</type></entry>
<entry><type>string</type></entry>
<entry>Convert &null; to "", numerical or lexical comparison</entry>
</row>
<row>
<entry><type>bool</type> or <type>null</type></entry>
<entry>anything</entry>
<entry>Convert both sides to <type>bool</type>, &false; < &true;</entry>
</row>
<row>
<entry><type>object</type></entry>
<entry><type>object</type></entry>
<entry>Built-in classes can define its own comparison, different classes
are incomparable, same class see <link
linkend="language.oop5.object-comparison">Object Comparison</link>
</entry>
</row>
<row>
<entry><type>string</type>, <type>resource</type>, <type>int</type> or <type>float</type></entry>
<entry><type>string</type>, <type>resource</type>, <type>int</type> or <type>float</type></entry>
<entry>Translate strings and resources to numbers, usual math</entry>
</row>
<row>
<entry><type>array</type></entry>
<entry><type>array</type></entry>
<entry>Array with fewer members is smaller, if key from operand 1 is not
found in operand 2 then arrays are incomparable, otherwise - compare
value by value (see following example)</entry>
</row>
<row>
<entry><type>object</type></entry>
<entry>anything</entry>
<entry><type>object</type> is always greater</entry>
</row>
<row>
<entry><type>array</type></entry>
<entry>anything</entry>
<entry><type>array</type> is always greater</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
<example>
<title>Boolean/null comparison</title>
<programlisting role="php">
<![CDATA[
<?php
// Bool and null are compared as bool always
var_dump(1 == TRUE); // TRUE - same as (bool)1 == TRUE
var_dump(0 == FALSE); // TRUE - same as (bool)0 == FALSE
var_dump(100 < TRUE); // FALSE - same as (bool)100 < TRUE
var_dump(-10 < FALSE);// FALSE - same as (bool)-10 < FALSE
var_dump(min(-100, -10, NULL, 10, 100)); // NULL - (bool)NULL < (bool)-100 is FALSE < TRUE
?>
]]>
</programlisting>
</example>
</para>
<para>
<example>
<title>Transcription of standard array comparison</title>
<programlisting role="php">
<![CDATA[
<?php
// Arrays are compared like this with standard comparison operators as well as the spaceship operator.
function standard_array_compare($op1, $op2)
{
if (count($op1) < count($op2)) {
return -1; // $op1 < $op2
} elseif (count($op1) > count($op2)) {
return 1; // $op1 > $op2
}
foreach ($op1 as $key => $val) {
if (!array_key_exists($key, $op2)) {
return 1;
} elseif ($val < $op2[$key]) {
return -1;
} elseif ($val > $op2[$key]) {
return 1;
}
}
return 0; // $op1 == $op2
}
?>
]]>
</programlisting>
</example>
</para>
<warning>
<title>Comparison of floating point numbers</title>
<para>
Because of the way <type>float</type>s are represented internally, you
should not test two <type>float</type>s for equality.
</para>
<para>
See the documentation for <type>float</type> for more information.
</para>
</warning>
<note>
<simpara>
Be aware that PHP's type juggling is not always obvious when comparing values of different types,
particularly comparing &integer;s to &boolean;s or &integer;s to &string;s. It is therefore generally
advisable to use <literal>===</literal> and <literal>!==</literal> comparisons rather than
<literal>==</literal> and <literal>!=</literal> in most cases.
</simpara>
</note>
<sect2 xml:id="language.operators.comparison.incomparable">
<title>Incomparable Values</title>
<simpara>
While identity comparison (<literal>===</literal> and <literal>!==</literal>)
can be applied to arbitrary values, the other comparison operators should only be
applied to comparable values. The result of comparing incomparable values is
undefined, and should not be relied upon.
</simpara>
</sect2>
<sect2 role="seealso">
&reftitle.seealso;
<para>
<simplelist>
<member><function>strcasecmp</function></member>
<member><function>strcmp</function></member>
<member><link linkend="language.operators.array">Array operators</link></member>
<member><link linkend="language.types">Types</link></member>
</simplelist>
</para>
</sect2>
<sect2 xml:id="language.operators.comparison.ternary">
<title>Ternary Operator</title>
<para>
Another conditional operator is the "?:" (or ternary) operator.
<example>
<title>Assigning a default value</title>
<programlisting role="php">
<![CDATA[
<?php
// Example usage for: Ternary Operator
$action = (empty($_POST['action'])) ? 'default' : $_POST['action'];
// The above is identical to this if/else statement
if (empty($_POST['action'])) {
$action = 'default';
} else {
$action = $_POST['action'];
}
?>
]]>
</programlisting>
</example>
The expression <literal>(expr1) ? (expr2) : (expr3)</literal>
evaluates to <replaceable>expr2</replaceable> if
<replaceable>expr1</replaceable> evaluates to &true;, and
<replaceable>expr3</replaceable> if
<replaceable>expr1</replaceable> evaluates to &false;.
</para>
<para>
It is possible to leave out the middle part of the ternary operator.
Expression <literal>expr1 ?: expr3</literal> evaluates to
the result of <replaceable>expr1</replaceable> if <replaceable>expr1</replaceable>
evaluates to &true;, and <replaceable>expr3</replaceable> otherwise.
<replaceable>expr1</replaceable> is only evaluated once in this case.
</para>
<note>
<simpara>
Please note that the ternary operator is an expression, and that it
doesn't evaluate to a variable, but to the result of an expression. This
is important to know if you want to return a variable by reference.
The statement <literal>return $var == 42 ? $a : $b;</literal> in a
return-by-reference function will therefore not work and a warning is
issued.
</simpara>
</note>
<note>
<para>
It is recommended to avoid "stacking" ternary expressions.
PHP's behaviour when using more than one unparenthesized ternary operator within a single
expression is non-obvious compared to other languages.
Indeed prior to PHP 8.0.0, ternary expressions were evaluated left-associative,
instead of right-associative like most other programming languages.
Relying on left-associativity is deprecated as of PHP 7.4.0.
As of PHP 8.0.0, the ternary operator is non-associative.
<example>
<title>Non-obvious Ternary Behaviour</title>
<programlisting role="php">
<![CDATA[
<?php
// on first glance, the following appears to output 'true'
echo (true ? 'true' : false ? 't' : 'f');
// however, the actual output of the above is 't' prior to PHP 8.0.0
// this is because ternary expressions are left-associative
// the following is a more obvious version of the same code as above
echo ((true ? 'true' : false) ? 't' : 'f');
// here, one can see that the first expression is evaluated to 'true', which
// in turn evaluates to (bool)true, thus returning the true branch of the
// second ternary expression.
?>
]]>
</programlisting>
</example>
</para>
</note>
<note>
<para>
Chaining of short-ternaries (<literal>?:</literal>), however, is stable and behaves reasonably.
It will evaluate to the first argument that evaluates to a non-falsy value. Note that undefined
values will still raise a warning.
<example>
<title>Short-ternary chaining</title>
<programlisting role="php">
<![CDATA[
<?php
echo 0 ?: 1 ?: 2 ?: 3, PHP_EOL; //1
echo 0 ?: 0 ?: 2 ?: 3, PHP_EOL; //2
echo 0 ?: 0 ?: 0 ?: 3, PHP_EOL; //3
?>
]]>
</programlisting>
</example>
</para>
</note>
</sect2>
<sect2 xml:id="language.operators.comparison.coalesce">
<title>Null Coalescing Operator</title>
<para>
Another useful shorthand operator is the "??" (or null coalescing) operator.
<example>
<title>Assigning a default value</title>
<programlisting role="php">
<![CDATA[
<?php
// Example usage for: Null Coalesce Operator
$action = $_POST['action'] ?? 'default';
// The above is identical to this if/else statement
if (isset($_POST['action'])) {
$action = $_POST['action'];
} else {
$action = 'default';
}
?>
]]>
</programlisting>
</example>
The expression <literal>(expr1) ?? (expr2)</literal> evaluates to
<replaceable>expr2</replaceable> if <replaceable>expr1</replaceable> is
&null;, and <replaceable>expr1</replaceable> otherwise.
</para>
<para>
In particular, this operator does not emit a notice or warning if the left-hand side
value does not exist, just like <function>isset</function>. This is especially
useful on array keys.
</para>
<note>
<simpara>
Please note that the null coalescing operator is an expression, and that it
doesn't evaluate to a variable, but to the result of an expression. This
is important to know if you want to return a variable by reference.
The statement <literal>return $foo ?? $bar;</literal> in a
return-by-reference function will therefore not work and a warning is
issued.
</simpara>
</note>
<note>
<para>
The null coalescing operator has low precedence. That means if mixing it
with other operators (such as string concatenation or arithmetic operators)
parentheses will likely be required.
</para>
<programlisting role="php">
<![CDATA[
<?php
// Raises a warning that $name is undefined.
print 'Mr. ' . $name ?? 'Anonymous';
// Prints "Mr. Anonymous"
print 'Mr. ' . ($name ?? 'Anonymous');
?>
]]>
</programlisting>
</note>
<note>
<para>
Please note that the null coalescing operator allows for simple nesting:
<example>
<title>Nesting null coalescing operator</title>
<programlisting role="php">
<![CDATA[
<?php
$foo = null;
$bar = null;
$baz = 1;
$qux = 2;
echo $foo ?? $bar ?? $baz ?? $qux; // outputs 1
?>
]]>
</programlisting>
</example>
</para>
</note>
</sect2>
</sect1>